Free with a plan just means you paid for it in installments without them breaking down how much of your monthly payment is going towards the device vs towards the network use. Had you opted for a cheaper device you could have got the same plan for less money. The phone is never actually free, just cleverly marketed to seem free.
I recently got one of these (Galaxy Z7 Fold) and I can't imagine ever going back to a regular phone. The big screen is what makes the phone finally begin to resemble actual productivity tool.
It was a prepaid plan that was the same price whether I got the phone or not. I guess you could say everyone who didn’t get the phone was subsidizing those who did, but there’s no way to opt for lower pricing if you BYOD. So no, in this case that’s not really true. If it were Verizon where you can pay less if you BYOD then sure but that’s not what I did.