I'd guess that either they think communism = all things bad (because they have no clue what communism is), or they think Trump is on Putin's side and they didn't notice that Russia hasn't been communist for 35 years now. The most charitable reading is that they think Trump is authoritarian and that the USSR was not so much communist as an authoritarian dictatorship despite its communist rhetoric. That last one is at least debatable, but it requires more nuance than is probably there in sticking a hammer and sickle in the middle of Trump's name.