They don't need to actually eat humans though. If one does so, it's just them being picky or cruel enough to consume fellow sapient species:
They are *meant* to be cruel human-eaters. Most of them at least.
And after a while Yukari returns them to Outside, where they will probably end up in retreat or, if they are lucky, forget about this nightmare.
Also youkai actually did eat humans, but not after establishing of Hakurei Barrier and especially not after establishing Spell Card rules.
Actually, Akyuu pretty much says that outsiders ending in Gensokyo are left alone without being helped, unless they manage to reach the village and settle there or ask for help to the Hakurei shrine maiden, but most of them get eaten because they aren't able to recognize a youkai (and even if they could, escaping would be basically impossible). Even PCB prologue says that almost noone manages to get back to their world.
Because honestly, why should Yukari bother to return them to the outside herself? She isn't a nice person to begin with (not for humans), and i see no reason for her to worry if a human from outside gets killed/eaten. They aren't important.
The SC rules only protect the villagers, not *every* human in existence. It's known that youkai go on hunting trips in the outside world to hunt humans, and the vampires of the SDM are known to feed on humans too.