Maybe there is a catch, but it is not clear to me why so many assume there is. Supermarkets and other stores have for a long time sold loss-leaders to lure a person into the store (the catch). On average, it works, but there is nothing preventing someone from going into the store just to purchase the loss-leaders. Yes, going to the supermarket and purchasing this "loss-leader" may not yet be an option (don't know), but we are closer to this than ever before.
I also do not see a 0% expense as a floor to fees. I think they could go negative.
I believe this is good a development and I am happy Fidelity is doing this. The prices for mutual funds have come way *way* down from when I started investing. Vanguard gets most of the credit for this, but I'll take the competition from any place it comes and if someone wants to leap frog Vanguard I'll cheer them on all the way.
I have no current plans of use these zero expense funds, but my institutional Fidelity funds came down in price as part of the roll out, so I'm happy.