Right from the DOJ's Technical Assistance Manual -
https://www.ada.gov/resources/title-iii-manual/
The only defense for these kinds of screening programs is the last line where they try to argue that it's necessary to provide the accommodation. And the argument against them is that the system tends to screen and discriminate even if unintentionally.
Places that normally provide services adhoc without prep are in a far worse situation... or accommodations that really don't require any burden on the business. Because they have a harder time justifying that the registration or screening is necessary to provide the accommodation.
This is where a business like UNI would try to argue qualifying people is necessary to maintain the accommodation.. but have a harder time doing so when the system flows right along with something they already do, or doesn't put unnecessary burden on them.
The standard is high that the process is necessary because the default posture of the law is the disabled should not face any additional burden to achieve equal participation.
But it's a standard that must be argued in court - because even the DOJ's guidance is constantly refined as more and more concrete examples are flushed out over the years. Examples today include standardized testing, ticket sales, etc. Areas that the DOJ has now written into specific guidance, but even the 'requirements' are open to interpretation because they are more principals than they are absolute quantities. But the guiding rule is the disabled shouldn't face additional burdens. So for instance requesting testing accommodations should be evaluated in the same time frame needed to register for the tests.
The theme parks accomodate guests who walk up and buy tickets without any reservations. They should be able to do the same with someone who needs an accommodation.