No its not being a purist at all. Legally, the law doesn't recognize any whataboutism whatsoever (unless its in the specific sense of legal precedents). So you can't argue your innocence in a bank robbery just because another gang also robbed a bank but did not get convicted.
The law is pretty clear; its just that most people are not prosecuted for it.
Sometimes you can. If the government knows banks are being robbed, has the opportunity to catch thousands of robbers, but only catches, say, female bank robbers, those robbers have a valid point in court, and may get free. Justice must be impartial.
It shouldn't immunize the guilty party, but it is useful to point out inconsistencies in applications of the law. Whataboutism is terribly useful for identifying the injustice of consistently punishing one party/race/gender/etc group while others get away with slapped wrists. We can argue about whether or not this is true, but decrying whataboutism misses the point.
The law is pretty clear; its just that most people are not prosecuted for it.