i think at least a lot of things (if not most things) that i pay for have an agreed-upon result in exchange for payment, and a mitigation system that'll help me get what i paid for in the event that something else prevents that from happening. if you pay for something and you don't know what you're going to get, and you have to keep paying for it in the hopes that you get what you want out of it... that sounds a lot like gambling. not exactly, but like.
In the US? No, you actually do not need to pay for the service if you deem the quality of the output to be substandard. In particular with art, it's pretty standard to put in a non-refundable downpayment with the final payment due on delivery.
You only lose those rights in the contracts you sign (which, in terms of GPT, you've likely clicked through a T&C which waves all right to dispute or reclaim payment).
If you ask an artist to draw a picture and decide it's crap, you can refuse to take it and to pay for it. They won't be too happy about it, but they'll own the picture and can sell it on the market.
There must be artists working on an hourly contract rate.
Maybe art is special, but there are other professions where someone can invest heaps of time and effort without delivering the expected result. A trial attorney, treasure hunter, oil prospector, app developer. All require payment for hours of service, regardless of outcome.
It'll mostly depend on the contract you sign with these services and the state you live in.
When it comes to work that requires craftmanship it's pretty common to be able to not pay them if they do a poor job. It may cost you more than you paid them to fix their mistake, but you can generally reclaim your money you paid them if the work they did was egregiously poor.
