The instructions do ask you very nicely to not be a jerk about abusing the free resources 
Also, it is quite hard to hit the limit, but if you are ̶u̶s̶i̶n̶g̶ ̶t̶o̶o̶ ̶m̶a̶n̶y̶ ̶o̶f̶ out and out abusing the free resources, they are running some kind of software that notices and squashes the running thread.
It must be really good at spotting it though, because I used the free version for over a year before I got to pay for it and never ran into the same problem in a real world use case.