You can! Nothing stopping you from buying, say, the latest iPhone at retail price without the postpaid subsidies.
The law also requires that carriers unlock any locked phones after the postpaid contract period is up (if customer actually paid it off): https://www.fcc.gov/general/cell-phone-unlocking
If you're suggesting that we restrict rights to optionally buy/sell subsidized phones and pay them off over a 12-month period, I can't get behind that. Let the people choose which financial model they want. No need to restrict postpaid programs just because people don't like the idea of locking during payback periods.
It does not sound ridiculous, because unlocked phones have been available for purchase at full price for over a decade in the US, and longer in most other countries.
No one has been forced to enter into a contract with a mobile network that stipulates their phone be locked to the network for a long, long time.
Except that is actually what is happening in reality. So yeaaahhh...
The take the price of the phone, divide it into 12, add that to your monthly bill until you pay it off, and unlock it when you are done with the payments.
Or, maybe, give a discount contractually bound to staying with the operator for a fixed amount of time instead of locking the phone forever?
It may sound outrageous but what about providing quality service so that customers willingly stay with the operator instead of keeping them hostages?