Most mountains I've had passes from will give you a day pass for free if you just forgot your pass. Forget it again though and they typically charge you, something like $10. They usually deactivate your card for the day, so you can't give it (or the lift ticket) to a friend. If I remember correctly, if you lose your pass they'll charge you $10-20 to make a new card each time, and again, they will deactivate your old card.
I imagine they do it to mitigate theft. Before they could scan cards to see if they're active, people would lie and say they forgot their pass. They'd get a day pass or a replacement, then just give it to a friend. Scanning cards helped stop that, but people can still get their cards deactivated and get a day pass and give it to a friend then play stupid when the scanner doesn't work or find a life without someone scanning. Limiting or charging for forgotten/lost passes is probably middle ground for mountains and riders.