It's worse. They're shifting the economic risk of shitty service off to the worker - even if/when much of the service is beyond the control of the worker.

For example: you deliver food for fictional service Yoter Eats. The customer pays a fixed price for the meal plus surcharges. Yoter Eats and the restaurant are guaranteed to get their cut no matter the quality of the meal, the completeness of the order (ie missing items 'accidentally' left out by the restaurant), how well it was dispatched, how quickly it was prepared, etc.

You, the delivery person? You're the only expense of the transaction that the customer has control over.

You have no recourse if your payment for services is reduced or withheld.

It's even worse: some services don't show how much the "tip" is before you accept the order (because they don't want you to refuse low tip orders) or after the transaction completes, other services take a portion of your "tip" and don't tell you how much they took.

Some services allow customers to "tip bait" - they can set one tip when they order, but then reduce it for no reason, up to hours after the order is delivered.