It may be simple to avoid, I myself don't have to worry about this as much as I'm on a byod sim prepaid plan, I can just buy an unlocked device, but the truth is that many phones on ebay are used phones, many of which were carrier subsidized. and used carrier phones are normally cheaper than the full unlocked versions. This move could kill a substantial part of the second hand market.
This is about greed, carriers can make it harder to switch by refusing to unlock your phone, even if your contract is up or you've left and paid the etf, which in reality pays for the rest of your subsidized phone not the mention the boom for phone manufacturers to increase sales, as more phones become harder to resell.
With T-mobile's refarm and bring your own device promotion. I fully expect AT&T to start refusing to unlock phones, regardless of your contract being over. Verizon and Sprint may be better about it, as their phones aren't easily moved from one carrier to another. It's anticompetitive and anticonsumer, they shouldn't have the right to tell you what you can do with a device you own.