All carriers don't do it on all plans (or even all postpaid plans).
T-Mobile has two sets of plans, "Even More Plus" (no contract, no phone subsidy) and "Even More" (2-year contract, $20/month more, phone subsidy of up to about $250). Maybe this will start a trend among carriers.
Don't like contracts and ETFs? Try buying a used smartphone on eBay or Craigslist (or you can buy the phone from T-Mobile at an unsubsidized price) and get one of the Even More Plus plans. (Of course, it needs to be compatible (GSM and on the T-Mobile frequency bands))