As long as you're providing the BBs at cost or with only a marginal markup (possibly combined with decreased field fee to compensate), I don't think it should be an issue. You're ensuring the safety of the field, and people don't have to worry about buying their BBs beforehand. If you add on the above mentioned BB storage for frequent players, the only hole I see is not being able to load mags until you're at the field (which is probably better for your mags). But really, that's about it. Storing BBs on your end may well simply involve a box, a sharpie, and some duct tape to ensure the bags stay closed. Sounds to me like everyone wins.
|: Gaudeamus igitur, Juvenes dum sumus :|
Post iucundam iuventutem, Post molestam senectutem
|: Nos habebit humus! :|
Raug, draugi, kā šī kanna mirdz, kā puto alutiņš? Dievs zin, cik ilgi līksmo sirds, lai baudām drīz!