I think you are over thinking the word "permission". Yes, asking "may I use your bathroom?" is more polite and less blunt then just announcing "I'm using your bathroom". Your friend is right. But its not real permission, because yes, the person is almost certainly going to say "of course, its down the hall to the left" or whatever. But ultimately in the polite dance of social etiquette, you are intending to use something of theirs, their resource, and yes you should ask first and not just declare "I'm doing this!"
If you really hate the idea of asking for permission, tweak it as many posters have said they do "where is your bathroom?" or "the bathroom is down the hall, right?", or even just a "please excuse me a moment". These are all questions/requests as well, although less overt. But its still paying forth the respect for someone's home that while yes of course they will let you use it, its still theirs, and its polite to ask first.
Another excellent post from WillyNilly.
You put your finger on what was bothering me a bit about this string. I don't think it matters a whole lot whether the question is, on its face, for permission or for directions. Either way is fine, but what is the problem with asking nicely even when you know what the answer will be? I just do not get the aversion to "please" and "thank you" and "may I?" that seems to underlie so many posts seeking to use them only when absolutely required.
See, I agree that the question "may I use ..." is a convention like "how are you?" -- not really so much a question or request for permission, in the sense that the OP is correct that of course there's no doubt that the answer will be yes. But the same is true for "May I please have a glass of water?" Of course the answer will be yes. But you don't just march over to the cabinet for a glass in a stranger's or acquaintance's home.
I stress that I am NOT talking about when you are in your good friend's or your relative's house where you know that you are welcome -- maybe expected -- to take care of yourself. And I also think that if you are at a big party and can see perfectly well where the powder room is, there is no need to go find and interrupt the hosts and ask them anything; just use it.
If you are in a home where you already know where the facilities (I'm not even going to chime on which noun to use -- I don't find any of them unacceptable) are, then you needn't ask anything -- but you excuse yourself politely (what "politely" requires will depend upon the circumstances; even "I gotta go to the can, man," can be okay -- but, like, when you're watching the game with your football buddies, but not at a dinner party at your boss's house).
Even in such familiar situations, and for obvious "yes" questions, what is so hard or bad about saying "please" and "thank you" and "excuse me"? Why try to say them as infrequently as possible? My husband and I say, "Please pass the salt" to each other. Doesn't cost us anything. This is pretty much the same issue I raised in the post about "I'm good" instead of "no, thanks." If you like to say "I'm good," fine, but say, "I'm good, thanks
" -- the same as you'd say "No, thanks," not just "No." Why not?
Likewise, I don't see much of a difference between "May I please use the restroom?" and "Where is the restroom, please?" IMHO, they are both fine. So is Goodness's ""Scuse me, where's your bathroom?" But "I'm going to use your restroom now" and even "hey, I'm going to use the bathroom, if you don't mind" are not, in my opinion. (The "if you don't mind" helps little, but the fact that it seems to be straining to avoid using the conventional question forms above would make me, as a host, pause for a second to figure out if there were some subtext.) And I bet that's what your friend meant, OP -- not that you must secure permission, but that, in your friend's opinion, "May I please ..." is the more polite way to ask. I would be interested to hear what your friend thinks of "Where is the restroom, please?"
(By the way, I do agree that hosts past Cub Scout age who think it is hilarious to say, "NO!" are being obnoxious. I once asked a middle-aged man, to whom I had just been introduced, where he was from. "My mommy!" he replied. He's actually an okay guy, but every time I see him I can't help remembering the moronic first impression he made by doing that.)