The developers could quite easily have implemented a counter of some kind should they wish to implement a limit and I certainly would feel no ill will were they to do that at some stage.
I, for one, given my circumstances, am very glad they they have left it open as they have - had I 70$ to chuck at it, I certainly would. When I do, I will.
Probably a few more sales, but at the cost of forcing some current users to stop using their product (if they can't afford the full version yet), or waste time jumping through anti-DRM hoops -- I've known people who are constantly setting their computer clock back, or tinkering with the Windows registry, or... all kinds of silly things so they can keep using an expired trial version of some software.
If they can keep those users on the trial version (and earn some goodwill as well for their flexibility), eventually some of them will be in a better financial or mental situation (whichever variable needs to change!) and will pay the license fee.
If they start the user down the path of battling DRM, or being forced to stop using their software, in either case they're starting a conflict.
So it's a business (and human) decision; they want to stay on your side even if you are pushing their internal definition of "continued use".