Because they want to bully someone, and tankies are acceptable. It's not any different from the tankies that kick people out of their book clubs for not worshipping a dictator from a country they've never even been in.

People only argue on the internet because they want to scream at someone - anyone.

I refuse to argue politics with people. It's better to ask a tankie "will you help me with my project?" They'll either say yes or reveal that they're a bully too.