Yes those people probably have homes, jobs, and a family and friends support network. With the homeless, they are missing more than one of those things, if not all of them. Is it even possible to have a prescription when you don't have a mailing address?
Not really in actual, regular, uninterrupted practice. There are so many problems with the US healthcare system at every level of it when you have excellent health insurance, especially in terms of regular maintenence medication for something as garden variety as asthma, that when you get down to the level of real emergency help absolutely medically required, it is usually a severely delayed and often interrupted swampy mess. I've been at both ends of this spectrum of care. Both are unreliable, but obviously, it is far worse-- a nightmare of indifference really, at the bottom. And many meds (like inhalers even) require ridiculously expensive, regular tests. The homeless and impoverished are often given less effective medication or ineffective medication to treat serious ongoing conditions.
You'd need to pay the doctor, and for prescribed drugs. Some doctors treat indigent patients at reduced prices, or pro bono. Also, food and drug chains often sell generics at discounted prices. And some US states prvide drug-discount services.
I guess the institution should be a transitory step in a social reintroduction process, not an asylum where to lock them away. (hopefully a system that does not chronicize them to assure funding streams.)
Bullshit. There are many people capable of losing touch with reality who live normal lives thanks to medication.