audi wouldnt miss only one car, they have a huge shit ton of money, they could just give away like 2000 cars and hardly see much effect, thats like saying your hurting wal mart or mcdonalds by stealing from them.  resorts like mammoth also have a ton of money, and the thing is, your not taking anything away from them, riding up the chair a couple times doesnt cost them anything. if you cant afford it and dont ski there, or if you find a way to sneak on the lifts so you dont have to pay, doesnt do shit, it doesnt hurt mammoth at all.
that being said, if you can afford it, just pay, its the nice thing to do, especially mammoth cause they dont seem like huge dicks like vail resorts.  but if you really honeslty cannot afford it and cannot ski unless you sneak on, then i think its fine.  really if i owned a big ski resort, and i knew a couple people a day were sneaking on i wouldnt care one bit.