If someone is an elected position, this implies that the majority of voters trust this person and let them do what they do, including the tax evasion.
First in in reluctant fairness to the current POTUS, my understanding is that the NY Times alleged tax avoidance not (criminal) evasion. There is a gray area between avoidance and evasion, of course.
Election does not imply or give anyone immunity, either moral or legal.
What election is supposed to mean is that the image you presented of yourself is the one the electorate in it's entirety has a right to expect you to follow.
It does not say "we trust you" and it definitely does not say "do whatever you want".
The outrage at these claims about the alleged tax avoidance of the current POTUS simply shows that most voters do not accept that being voted into office gives you either legal or moral immunity.
If you claim you're fair and honest in paying tax and it turns out you spend more on accountants and lawyers to avoid paying it that you did pay in taxes, that will, not unsurprisingly, lead to all those people who cannot even afford an accountant (or rent or vacations or medical cover) shouting "foul" really loudly.
People have demonstrated by complaining that, no, being elected does not endorse bad behavior.
Regarding Trust. Gallup published a survey that shows that (by 2016) trust in US politicians had fallen steadily from 68% in 1974 to a miserable 42% in 2016. So it is clear that far from trusting politicians voted into office, there is a clear trend to distrust them.
Note that the trends cross party lines. Democrats don't trust democrats and Republicans don't trust Republicans and both have very similar trends down.
Good healthy cynicism. Well done, America - a step in the right direction. :-)