which limits your liability to the amount pre-loaded on the card.
It limits your maximum possible liability, BUT it also creates a minimum liability, ensures you lose at least something --- such cards have a load fee, so you are guaranteed to lose X% of your money deposited, without it being spent towards useful purchases.
Also, the amount on the card is unlikely to be an exact multiple of amounts spent -- e.g. You have $10.00 on the card, but you need to spend $9.11 for the in-app purchase., so you have amounts like $0.89 left on each card you use, that becomes dormant and eventually gets lost.
Then there's the whole "inactivity fee" thing. If you go too long without making purchases, then, you start losing deposited balance.