stores and drink it in their apartments or not, it has no bearing on what is sold in the stadium (at least most of the complaints about the "no demand" statement seem to focus on its lack of availability in LES). If the students in the stadium who want caffeine go ahead and buy what's offered, they are showing they are willing to substitute one good for another. That's signaling that demand for the existing selection is sufficient and they don't need to offer other drinks and risk offending some.
When enough fans stop buying what is offered and prove that there is a demand, then BYU can still use that line - regardless that they've never really done standard market analyses to determine that. I'm positive BYU's reluctance to offer caffeinated drinks has its roots in the "can't drink caffeine" standard that was in the church for many years and still lingers with many.