Lol It's very difficult to convince the Verizon fans ... Same applies other way as well, very difficult to convince the carrier haters. Everyone has their own opinion let's leave at that.
My theory:
To start with yes I know just having the right radios is enough for an already active sim to work on Verizon. For new activations you need Verizon blessing but in HTC 10 case, the radio isn't there which can be flashed and get the already active sim working. One could argue they could have had the radios to work for all carriers, but we don't know what deal HTC had to make for Verizon to carry HTC 10.
1. For latest Google phones - it took sometime for them to get it working, but Google being a big player got to some terms with Verizon and have the unlocked model activate on their network with Verizon blessing.
2. Motorola - Duh ... Droid they obviously have a great relationship, make them whatever droid branded phone Verizon wants, put whatever Verizon wants, stamp the logo wherever Verizon wants. Given the good relation Motorola could get the pure edition work on their network.
3. LG, Samsung - they don't even try or care.
4. HTC - they tried with One A9 and definitely couldn't get Verizon to agree. Initially when it was released they said Verizon support coming to unlocked A9 later but gave up after a month or two. So whatever talks, certification, backdoor deals or whatever it takes for Verizon to not get pissed didn't work. With the HTC 10 it's the same, but this time they got at least the Verizon specific version.