It's a feels good story, most of the time it doesn't make sense. The author probably doesn't know how to make a meaningful enough connection between the elf and the mc that they could stay there. So they went the easy way
yep ofc obviously it's going to jump into slavery like it's basically common sense at this point(sarcastically speaking) and mc is like "i can't say no because i'm a pure and kind person so if that's what they want then they shall be" lmao