I thought garlic fries, like the sort at Giants games, were delicious! Some garlic, olive oil, parmesan, etc. I was shocked to discover that here and in the UK garlic chips consist of chips (fries) with garlic-accented mayonnaise spread all over them.

And don't call it Aoli. That's just dressing up a disgusting condiment with a fancy word.

Since then I've looked at the ingredient list of every other product with great suspicion, expecting to find mayo everwhere...

I use butter and mayo for one of two things: flavour (eg I love kerry gold on toast ;-) often nothing else) or moisture - but if I don't need the moisture, usually because I've got tomato, peppers, coleslaw, relish or something else providing that, I will leave butter and mayo out. Unless I want them for the flavour. (As an side, I don't understand why people would ever want butter on a breakfast roll)

These are precisely the instances in which I would want something oily (butter, marg or mayo) as waterproofing for the bread. (Mustard also works, if it's appropriate for the sandwich.) "Everyone has the gout," as they (don't actually) say in French.

I just don't need a peanut butter and butter and jelly sandwich.

Of course I also grew up in very immigrant heavy areas so that may have been part of it.

In Russia, where I grew up this is the only way to make a sandwich.

In the UK, standard practice is to butter sandwiches on the inside, ie on the surfaces of the bread which interfaces with the filling. Are you by any chance talking about buttering the outside?

I think a lot of it comes down to the fact that most sandwiches these days are prepared and then eaten right away, whereas I was brought up in a world where sandwiches tended to be prepared long in advance of being consumed.

That said, Americans will cheerfully slather "mayonnaise" on bread.