If two (or more adults) agree about their roles as Feeder/Master and Feedee/slave - why shouldn't they just live it??
That's fine, but that's not the Evil Feeder fantasy.
An Evil Feeder does not give his victim a choice. He kidnaps her, imprisons her, and enslaves her.
This is illegal everywhere (as far as I know!) And immoral, as you're taking away someone's freedom and taking them away from their family and friends. And impractical, as it's very hard to take care a of a prisoner like this. What about dental care, health care, nosy neighbors?
This is why the Evil Feeder fantasy, while physically possible, will usually be and should always be just a fantasy.