I've had to fight dragons in Riften, Dawnstar, Falkreath and Winterhold, so there's really nothing stopping them from getting into the other cities and towns.
I don't think the OP meant for Dawnstar, Falkreath and Winterhold to be included, they're not seperate from the world cell.
Is it intentional that Every time I travel to Winterhold and Dawnstar, as well as if I fast travel to Riften Stables, there is ALWAYS a dragon there circling? It never fails, every single time I go to either place there's a dragon there, and I'm really getting tired of killing them....
Do you fast travel? IIRC, fast travelling has some affect on dragons.