I hate to do it, because it seems like such a cut-and-dry situation, but I'm going to chime in with...
Assume Good Faith, and There's Probably More to the Story. Perhaps when the group formed they SPT'ed and identified immersive RP as their first 4 priorities. Perhaps the GM had no idea what veganism's about and had no idea it'd wreck that person's system. Perhaps.... Probably not, *as described*, but perhaps. We're Only Hearing One Side.