Having Fidelity support Bitcoin, Dogecoin or Ethereum is about as difficult and expensive for them as supporting a new stock or commodity exchange, and about as risky... as long as they don't act as a market maker. They'll almost certainly make money on fees.
Meanwhile they get the marketing value. They spend over $100MM annually on advertising (by way of a quick googling, I have no special insight) so it seems reasonable that they like being talked about, especially by people who might otherwise be avoiding them.
