Sam2, you’re correct to take a different approach because the pasuk is stam not a rayah either way. A yeshiva guy shouldn’t apply derech eretz from what was appropriate 2000 years ago for soldiers in the wilderness, during a war or in a survival situation. Similarly, I never saw a yeshiva guy take home an aishes yefas toar from the grocery store… heheh.
So, I’m not even 110% sure that I heard it, (it’s possible that it was only raised as a question but that there wasn’t an affirmative answer) but on the 99% chance that I did hear it, it was NOT that it’s assur, it’s that it is beneath the dignity of a recognized ben Torah to bend over a fountain to drink, and that it’s more appropriate to use a cup and drink like a mentsch. Also, this wasn’t said in context to yeshiva high school guys or young beis medrash bochurim, but intended to refine the sensitivities of the altar bochurim who would soon be getting smichah.
In a similar vein, I have always heard (this time 110% sure I did) that it is inappropriate for a ben Torah to drink beer from the bottle/can, but rather he should pour it into a cup. I don’t recall seeing this in tshuvas anywhere either. Just like drinking from a fountain, it’s just not one of those things that’s a “halacha” – assur or mutar- but rather a refinement of behavior that applies to those who should be sensitive enough.