It doesn't dirty your lap if you do it right, therefore, as far as the game is concerned, it is not "cheating". People can argue about "the spirit of the rules" all they like but the in-game timing system doesn't care for such things.
The issue here is that when Turn 10 developed the track they weren't 100% thorough with defining clean/dirty lap areas, and there are similar issues on a number of circuits (Hockenheim, Laguna Seca and COTA are the first 3 that spring to mind). In a race these things don't really matter but when it comes to Hotlapping every minor advantage counts and
will be exploited by those that are
playing to win.
The solution here is for Turn 10 to go through each circuit and adjust exploitable corners wherever possible, not for everybody to play to some honour system where some will make use of what the game allows them to do and others won't. Whether or not that's worth their time however is a different story, as competitive Hotlapping only appeals to a minority of their playerbase.