There have been news stories recently about an FBI agent assigned to the Russia probe. Apparently they wire fired from it by Mueller because they had sent messages to another person that were highly critical (and insulting) of Donald Trump.
What is wrong with this? Are American FBI agents not allowed to have personal opinions? Or is it politically motivated (was Mueller foreseeing that Republicans might use that to discredit their work) ?
FBI agents are particularly restricted in what they may do or not do. This is partially specified by law, and partially a professional norm. Both law and custom allow FBI agents (as well as all other governmental staff) to have opinions. However, they must be careful about how they express those opinions.
The United States Constitution protects the ability of all people to engage in political speech freely. However, federal employees are in a tough place: while they have the right to political speech, they can't use their office to push their (personal) agenda.
The Hatch Act governs what kinds of partisan activities federal employees may participate in. In general, civilian federal employees (except a few, such as the President and Vice President) may participate in public life with a few exceptions:
Within the Hatch Act there is a list of agencies labelled "further restricted". Employees of these agencies are especially restricted in the kinds of partisan activities they can pursue.
For example, while all federal employees may not actively campaign for anyone under certain circumstances, further restricted employees may not actively campaign at all. This means they may not make public statements supporting candidates, parties, or platforms, make those posts on social media, distribute campaign literature, etc.
Also, further restricted employees may not:
Hold office in any political organization (including political parties, but also politically-oriented community groups).
Encourage subordinates (even tacitly) to attend partisan events.
On the other hand, the Hatch Act explicitly protects the rights of federal employees (even 'further restricted' ones) to express their opinions on politics. However, that commentary cannot be either coordinated with a partisan group or aimed at actually influencing an election.
The Federal Bureau of Investigation's employees are further restricted. Being critical of the President is not a restricted activity - in fact it's protected (as the ability to express opinions).
Even though the Hatch Act does not prevent an FBI agent (or anyone else) from being privately critical of the President, there is a set of professional norms which make it unpalatable. In many agencies, it is equally important to avoid the appearance of partisanship as well as actual partisanship.
This is partially based on my experience: I work for a state agency. Even though we are not covered by the Hatch Act, our internal policies are almost identical. And the reason is easy: in practical terms, the appearance of being biased would compromise our role in government. Similarly, even if the agent-in-question really is entirely unbiased (and non-partisan) the appearance of being partisan may be risky enough to justify removing them from that team.
FBI agents are granted considerable power in the execution of their duties. As such, they are held to higher standards than average people, and are expected to execute their duties in an objective manner.
The agent in question is perfectly free to have whatever opinions they wish. What they cannot do is act in a manner that casts doubt upon their impartiality and dedication to establishing fact. Not if they wish to remain part of a potentially devastating investigation.
This is not limited to FBI agents. General Stanley McChrystal was fired from the Army for making remarks critical of then commander in chief Barack Obama, despite McChrystal's record of being very effective.
There is a difference between having a personal opinion and having a personal opinion while having a personal involvement with the subject matter of an investigation.
For example, let's consider a hypothetical FBI agent investigating a suspected drug dealer for supposedly being a kingpin of a marijuana empire (and I am using a soft drug such as marijuana deliberately here). Now let's consider what should happen if it was discovered that the same FBI agent was politically active in the movement to legalize pot (organized letter writing campaigns, etc.), and it was found that he was an investor in "High Times" magazine. There would be a really good reason not to allow him to continue to be part of the investigation.
While this level of detachment is a little more difficult to ask when it comes to political candidates (because everyone has some political views), it is the voracity of those views which may make an agent a bad choice for a unique investigation targeting a particular politician or a political candidate.
If the views are provably strong enough that an agent would not make a credible witness in a courtroom, when speaking to a jury (because his partiality would be suspect), this makes him a bad choice for this case even if it is expected that he would never have to address a jury. The agents' professionalism generally deserves a benefit of the doubt and their ability to put their personal feelings aside is something that is taken for granted. But if those feelings are strong enough, such benefit of the doubt may no longer be warranted. If there is doubt, in the interest of accuracy of the investigation, it is obviously better to err on the side of caution.
