how do u use a fps engine in an rts?
All game engines have been relatively similar for some time. It's just the perspective and the optimisations. You could argue that there is nothing such as an fps or rts engine, they're not mutually exclusive. Unreal technology has been used for all sorts for ages.
It's good business for developers. Game engines and their tools take years to produce, and then the game content on top of that, you're looking at a huge investment and one way to make that back is by licensing.