From Twitter's perspective, meme-makers aren't the enemy. Drama is good for their revenue numbers. Bulk fake accounts are a problem in general, so I'm sure they noticed the spike. But I'd guess the false election information is the honest motivation for the mass ban. People have basically given them and Facebook a mulligan on the 2016 election. But they definitely don't want to be blamed twice an a row for ruining democracy; not only does it make them look careless, but it significantly raises the risks of intemperate regulatory action.