https doesn't care about the content, it's the browser that tells the user that his communication is in the clear, and there's no assurance as to who the user is talking to.

> What danger could possibly happen if I'm reading about a Physical Therapy clinic?

Depends what is a "danger" to you. Your insurance learning you're having issues and deciding to increase the amounts you owe them, because they saw that your back is aching, is definitely a problem.

> But unless the Physical Therapist knows how to manage the server, they get this scary warning.

Wrong. In 2020, if the Physical Therapist can have an http website, they can have an https website with a valid certificate.

It's the same for your grandma store. Going from no website to http is a much much bigger step than going from http to https.