This is true, but I never seen any evidence of it over the years. At one time At&t used a tactic called FUD (Fear, Uncertainty, and Doubt) to try and convince and many people as they could to not tether. They weren't really going to charge their customers as it would piss too many of them off. Using FUD would work for them to an extent because they could cause a % of non authorized tethers to be too scared to do it anymore. The built in wifi hack is a little different because many are unsure how easy it is detectable compare to using a third party app. I beg anyone to go find evidence of Verizon billing customers for the use of unauthorized tethering. I've seen some claims such as someone getting billed $80,000 by verizon or some garbage like that, but the handful of people could never produce anything close to being evidence.
