Arial Black
Adventurer
I'll be the first to agree that there's a lot of gray areas in 5E
Sure, and it's the DM's job to decide if this bit of grey is actually black or white for this roll. When the player asks if this roll has advantage, the answer is either black or white for this roll, even if the general case may be grey.
What you are doing is taking a piece of the rules which is black and white, pretending it is grey, and then ruling that it's purple!
Just because some parts of the rules are grey (stealth) doesn't mean that every part of the rules is grey!
DM: What's your AC?
Player: ...It's a grey area, really. Don't you think?
DM:....NO!