Well, you can give them your own business requirements, which should be high-level and state "I need a system that performs these functions for this many people

Well, you can give them your own business requirements, which should be high-level and state "I need a system that performs these functions for this many people". What you should not do is dictate design and implementation details. That's when you rely on their expertise. . .unfortunately, as you stated, that's rarely how it works. Also, it would have really helped to have someone - likely not a developer - to bridge the gap and ask the customer "do you really need people to set up user access before viewing plans or you just want that? why?" and then turn to the developer and say "ok, what does that mean technically? what the pros and cons?"