Advertising is indeed covered by the first amendment. However, various US Supreme Court decisions have also confirmed that the government can make restrictions on misleading commercial speech and that, in general, commercial speech is less protected than speech more broadly.
This is called "negative externality". Another example is junk food. It does affect everyone when people are unhealthy (health care costs for instance).
> It is so strange that this is allowed.
I think the US have ruled that advertisement is protected by freedom of speech. Actually some people get extremely defensive when talking about restricting advertisement (nanny state argument...).