It may depend on the state you live in. Many if not most mandate a trial period. But perhaps you are unlucky and live in a state where there is no such law. You might want to contact your state board and ask them.
You should also be aware that many dispensers/audiologists work on some form of commission/bonus (or profit if they are the practice owner). After your free no obligation test, if you make it clear that your decision to buy hinges on getting a trial period or you walk away, they will probably offer one. Make sure you get a written commitment to this effect, including your restock fee if any.
Most factories offer a 90 day sale or return policy to the dealers. So they don’t even get billed if they send them back to the factory in that time. But no one likes to make a sale and then lose it again, which is why so many of the audiologists you spoke to were not that keen to offer it.
Despite the attitute from those that you spoke to, any good hearing professional will offer a trial if pushed. And all good specialists will give you a ‘test drive’ in the office. I firmly believe that those who refuse a test drive in the office are just bad at their job. Certainly if I detect a hearing loss, I make sure that patient gets to listen to a set of digital hearing aids before they leave my office.