When I wrote "cheat," it may have had some bad connotations (go figure), and sometimes it is out-and-out cheating; but that's rare. More often, it's house rule/ DM acquiescence (as pointed out in this comment and Umbran's).
This is something that has been baked into the system since (IMO) 1e, which required certain classes to have extremely high minimum scores for entry, and made it extremely difficult to increase you ability scores (ioun stones, other certain magic items, occasional weird events). Moreover, players wanted to play "special" characters (you know, heroes!) and gameplay advantages didn't start accruing for abilities until 15 (16 for strength!).
So, again IMO, there has long been a ... relaxed culture regarding die rolling tracing back to 1e.