This has been my understanding with all the three mobile carriers. The reason you sign a term contract is to pay off the phone you received up front for no cost, or at a reduced rate. They lock the phones to their network until the contract is fulfilled, or cancellation fee paid.
Whereas mobile services that operate on month-to-month basis don't give a free 'unlocked' phone, as you could simply go to another carrier.
I stopped signing mobile contracts over 15+ years ago and either bring my own phone, generally bought from Ebay, or buy carrier's phone outright. This is why Visible owned by Verizon can charge $25./month with unlimited 4G/5G data/calls/texts for mobile service only.