The only possible thing inhumane about what is suggested from raising them outside is feeding them rabbit pellets because they are dry etc., but Rabbits still eat real food as well.
Just FYI-
Pellets (or at least, good pellets, not grocery store pellets) are considered only a supplement of the diet for small mammals like Rabbits to ensure they receive necessary nutrients, like calcium. You're supposed to feed LOTS of fresh, clean vegetables and allow unrestricted access to grass-based hays. Although, most pet rabbits get the "tiny pet-store cage with nowhere near enough room, no hay, and cheap, crappy pellets" treatment, which is horrible.