Because, when you wear a uniform, you represent the organization that issues the uniform, even when you're off-duty. When you act unprofessionally while wearing a uniform, you convey the impression that those who wear the uniform are unprofessional --- exactly the opposite idea that a uniform is intended to convey.

This isn't unique to TSA, of course. Any uniformed service has rules about what one may do, and not do, while wearing the uniform --- precisely for this reason.

If TSA wants to make a big deal about demanding respect for its employees, and try to encourage that respect by creating uniforms which look like those worn by law enforcement, then its employees wearing the uniform need to act respectfully when wearing them.

And having said all that ... yeah, it's probably not a big deal.