You're assuming that every client and every provider knows their actual HIV status. Because of the stigma that HIV presents, an overwhelmingly large population of people don't actually know their status. High-risk groups include those who have sex with multiple partners, and aren't aware of their partners' HIV status. That's basically every hooker and john out there because every hooker and john could potentially be lying (whether the lying is intentional or not isn't of consequence in this matter).
Sex workers and those who seek the services of sex workers are not the general public. We are a high-risk group. Porn stars don't qualify because they're required to test by law (and even that is flawed, as we saw a few years ago with the kink.com HIV issues). Lots of people actually advocate for PrEP use in porn as well.
High-risk sexual behavior, as defined by most medical institutions, including the CDC includes:
-Unprotected intercourse without male or female condom use, except in a long-term, single-partner (monogamous) relationship.
-Unprotected mouth-to-genital contact, except in a long-term monogamous relationship.
-Early sexual activity, especially before age 18.
-Having multiple sex partners.
-Having a high-risk partner (one who has multiple sex partners or other risk factors).
-Having anal sex or a partner who does, except in a long-term, single-partner (monogamous) relationship.
-Having sex with a partner who injects or has ever injected drugs.
-Exchange of sex (sex work) for drugs or money.
I don't ask my clients about their medical histories or their relationships or how many other providers they see. Because it's not my business. The fact of the matter is we as hookers and they as johns engage in high-risk sexual behavior simply by doing what we do. Most of us accept these risks and mitigate them how we see fit, and if someone wants to be on PrEP in order to make them feel safer, they should do their research and talk to a doctor