Earlier quoted context omitted.
Why not? They are enemies, not friends. We should vilify our enemies and celebrate our allies. This is the nature of the world. Sometimes you must draw a line in the sand and pick a side. It is not possible to straddle the middle and try to apply the same sets of rules to every actor; each circumstance is different and should be treated with the nuance it requires.
The thing is, if you treat all the russian people as enemies, than they will in fact all become your enemies. If some oligarchs were (secretly) opposing Putin before - then this is the way to bind them to Putin. "It is not possible to straddle the middle and try to apply the same sets of rules to every actor" And this would be, how justice is supposed to work. Apply the same rules to every actor. Otherwise it is not…
Let's be extremely clear: Russian people are not the enemy. Very few people here in Europe think this, and those who do are mostly the xenophobic assholes who enjoy thinking anyone who isn't them is the enemy anyway.
Oligarchs are Putin's keys to power. Sanctioning them is a way to hurt Putin's power and influence, which has a chance to bring the war to an end. Not sanctioning them because "oh there's a chance maybe one of them actually doesn't like the guy" is an idiotic move.