Recent media and blog coverage, including the February 10, 2009, AdAge.com post “Vitaminwater Runs Afoul of NCAA Banned-Substance Rule,” incorrectly states that student-athletes should not drink six of Vitaminwater’s varieties or they might test positive for banned substances. 

In fact, normal daily consumption of any of the 13 Vitaminwater varieties will not place a student-athlete at risk for testing positive for banned substances. 

Eight of the varieties, including Revive, the only NCAA-branded variety featured at NCAA Championships, contain no impermissible or banned substances and may be provided by member institutions to student-athletes. 

Three Vitaminwater varieties (Power-c, B-relaxed, and Balance) contain ingredients that categorize them as impermissible under NCAA extra benefit rules.  As such, schools cannot provide these varieties to student-athletes as a nutritional supplement, but they can be purchased and consumed by student-athletes on their own without any risk to their NCAA eligibility. 

The NCAA probably doesn't want to get in trouble for heart trauma related issues with the ingestion of guarana, taurine, caffeine and other additives those Vitaminwater varieties have, but use the "banned substance" or "unfair to competition" line so as to not publicly say something wrong about a sponsor... I'm just spitballing, but it's the only thing logical I can think of for a very illogical situation

It might also be that the levels of B/C/etc in those drinks qualify them as supplements.  Thus it would be a benefit for the schools to buy them, but nothing wrong with the students buying them. ( Thus the extra benefits rule mention.)  It would probably be the same thing for several items found at GNC.