They banned him after his Presidency. He did greivous harm and acts of hate before that, but only now are Twitter giving it the attention it deserves. Anyone else would have been banned long before now. He has incited a great deal of violence with racist dogwhistles, and only now he's facing any sort of punishment. The argument of 'free speech' falls apart when you factor this in.
I don't think twitter's tactics are as nefarious as you make them out to be.