It's a fair point that (1) a setting or story gets more interesting if magic follows actual rules (e.g. Brandon Sanderson) instead of going "LOL it can do anyhting 'cuz its MAGIC!!1!"; and that (2) it is certainly the GM's job to override RAW at those points where RAW gets ridiculous.
So yes, I agree with you that a ghoran shouldn't be able to feed itself. Doesn't change the point that in most campaigns (that don't involve multi-day underground exploration) it's a fine and feasible race to play.