It's not so much about the legality of the move though. If a player were to start blatantly time wasting, making cocky gestures, or juggled the ball idly in the middle of the game it wouldn't be taken well by players/fans even if it's legal. The principle here is the same, only what Neymar did is less extreme and the Athletic players did overreact. I'm just saying that I understand why it isn't taken well, especially when the situation is tense and players are frustrated/combative.
And let's not play dumb when it comes to Neymar, he isn't some angelic little victim. He is a known wind-up merchant with the gestures he makes, his history of diving, and general demeanor. Barca fans get angry when guys like Muller/Ronaldo show arrogant attitudes, Nerman can be guilty of the same. Just look at this:
Is it really hard to understand why opposition fans don't always like the things he does?? He shouldn't be discouraged from using his flair or doing skill moves. Doing skills like these (rainbows or the standing over the ball crap he does) aren't necessary at all but if he wants to do them, of course he should still be able to. He should just know that it won't always be taken well by players/fans and that it could start conflicts. What happened today was definitely an overreaction though.