Sure but using the past tense implies that the thing does not happen anymore. That's why the Mitch Hedberg bit is funny (I used to do drugs... I still do but I

Sure but using the past tense implies that the thing does not happen anymore. That's why the Mitch Hedberg bit is funny (I used to do drugs... I still do but I used to, too) is humorous. It subverts your expectation that the thing you "used to do" is a thing you no longer do. The US used to do horrible things to Natives... it still does but it used to, too.