I think the communist party of Russia is still centered around the radical left wing political ideology of early USSR, and is a relatively large party because the USSR did make a lot of true, if aging, believers and its left wing ideology does appeal to a substantial number of the young. They are a relatively resource poor grass root political fraction.
I think United Russia tolerates them because being 2 or 3rd largest party in the Duma they keep seats out of the hands of other parties, while their prospects for gaining much more seats is not good, so they don’t constitute a serious threat to the United Russia. The United Russia party also periodically pay enough homage to the USSR that the communists don’t quibble with the policies of the United Russia on most fronts, attacking them only for corruption. And ironically, lack of openness.
I think United Russia tolerates them because being 2 or 3rd largest party in the Duma they keep seats out of the hands of other parties, while their prospects for gaining much more seats is not good, so they don’t constitute a serious threat to the United Russia. The United Russia party also periodically pay enough homage to the USSR that the communists don’t quibble with the policies of the United Russia on most fronts, attacking them only for corruption. And ironically, lack of openness.