Did you even read that sentence?
I can chop off context too!Wow guys, look at him. He says you shouldn't be allowed to have fun!
That sentence states that you should not buy a license if you have no verified that WinRar will completely function on your computer. It does not state any obligation to purchase the software after the trial is over.
No, it clearly says he promises to if yes. This is not a simple matter of inversion. In simpler terms...
IF it works on your computer, you DO PROMISE to purchase a liscense.
Promise does imply an obligation, in fact. An obligation only met if it runs, sure, but I doubt many people have issues running WinRar.