Sorry. There was supposed to be a smiley face there (I still can never remember the difference between those two dishes anyway).
I don't make your distinction, but can totally see why one might (or why a nation might). For no good reason, I tend to think of tarts as having a non-pastry/non-standard pastry bottom (like cookie/biscuit crumbs or shortbread). But, the aforementioned banoffee pie (so good) has a biscuit crust and it is call pie.
And, while we are at it, why is it a graham cracker and not a graham cookie?