How will this help Twitter turn a profit?
A) Twitter has a bad reputation for bullying and harassment; almost any financial analysis I read about Twitter has this as a strong negative. If Twitter is cracking down on this finally, this would help this reputation and potentially it's profit.
(Note that I'm assuming here that Twitter actually is suspending these accounts not because they are alt-right, but because they are bullying or trolling. I know the alt-right has a bad reputation in some sources for engaging in trolling behavior; I do not know whether this applies to the individual accounts in question.)
B) Being a haven for extremely divisive speech tends not to be the type of place that attracts advertising from companies with large amounts of capital; companies like these (larger ones) tend to default to inclusive for obvious reasons (it's better to have a potential customer base of everyone).
I don't like the idea of banning alt-right accounts for the sake of them being alt-right, but I can easily see justifying bans if either a small population harasses a larger portion of the users, or if significant advertising dollars are at sake. Twitter is a business.