How do you get hit with this fee? I've not done any financing, but I've updated iPhones several times on Tmobile (2013-2016) and Verizon (2018-2019) without even having to tell the carrier...
If you go into the Apple Store and use the financing option then they will register your phone as a convenience to one of the big 4 carriers. I imagine you could finance and "activate" it later yourself and maybe avoid the fee if you have a physical sim card. But the new phones are eSim so that may play a role. I was told I could not participate in the iphone upgrade program if I wasn't on one of the big 4 carriers. I was on a big 4 but then I went MVNO with no issue. I haven't upgraded since tho.
Maybe I'm wrong about Tmobile, I can't find anything via searching about it, but Verizon and ATT of course charge it. I thought I had heard Sprint and Tmobile do too since the SIM comes with the phone and they have an excuse to charge, but they might be nicer than Verizon and ATT.