If you used the program once during the trial period then came back to it again for some reason or another after the trial expired, the trial worked exactly as intended: You want to use the program again, so it's asking you to pay up.

I don't see the problem here other than "I don't want to pay for software." which isn't the programmer's concern. If you don't want to pay, the programmer likewise couldn't give a damn if you can't use his program.