Hmm... I don't play in open any more, but when I did, I accepted the consequences of doing so, painful insurance screens included - that just seems right to me. Logging out to main menu to avoid being killed isn't honourable, I agree. I can't really agree that it's cheating, though.
The fifteen seconds of free damage you can inflict is what FD seems to feel is a fair punishment for exiting combat early, whether that exit is for urgent RL issues that have to be dealt with, or simply to avoid death. You might disagree with that punishment for being too lenient, but it's a painful stretch to call it cheating, when it appears to be a fully intended thing by the developers.
It seems even less like actual combat logging, which is something I believe the developers have described as cheating - and which might presumably bring down a shadowban at some point.