EDIT: I completely misremembered the title question while typing this answer. I was responding to "why is the status quo what it is" and not the actual question. Which is why it absolutely doesn't make any points in that regard.
The ones that would have to forbid the sidejobs are the ones profiting off them
That's the reason why it hasn't been changed, because with that huge a salary and the insane potential for abuse / legal bribery there isn't really any conclusive argument.
All the other answers here are perfectly valid, but are far outweighed by the negative implications of the current system, if one were to approach the topic objectively and neutrally.
But that will likely never happen, as usual with rules concerning the rulemakers.