Some TSOs are checkpoint-only.
Some TSOs are baggage-only.
Some TSOs, like myself, are dual-function. I'm trained and do both.

The polo shirts are designed specifically to be worn in checked baggage, yes, but also specifically to be worn outside of the view of the public. If the airport has a pre-check-in location, with the screening equipment in front of the counter, they'd have to wear the normal uniform. Only when the screening is done "behind the wall" can the FSD authorize the TSOs to wear the polo shirts.

Incidental views of the public, like when moving from checked baggage screening location to a break room, and stopping in at a gift shop to pick up a 20oz on the way there, is okay.

But you're not supposed to be seeing people in polo shirts operating the security checkpoint :P Here in HSV, if you wear your polo shirt because you know you're in checked baggage, and they need you upstairs to help with a rush, you have to swing by the break room first, change uniforms, and then report to the checkpoint.