If being kept in a cage is slavery then wouldn't prisoners of war; and jailed dwarves constitute slavery?
Prisoners are legally prisoners, either as losers of a military conflict, or as punishment for a crime/awaiting justice. Capturing peaceful civilians does constitute slavery.
# slave - a person who is owned by someone
# slave - someone who works as hard as a slave
# slave - work very hard, like a slave
# slave - someone entirely dominated by some influence or person; "a slave to fashion"; "a slave to cocaine"; "his mother was his abject slave"
without being directly owned or forced into labor; while also being looked after and kept from danger?[/quote]Keeping a lving being in a cage counts as owning, check with your local justice department if in doubt. Slave-owners typically used the they-are-fed-and-kept-safe argument to justify slavery. Basically, if you need a restraint, you're keeping someone captive. Presuming you are not insane, you are doing it for a reason, and keeping someone captive for your own benefit is slavery.
Since elves need freedom of movement, contact with sunlight, plants, etc. you're arguably torturing them as well - instead of meeting their needs.
There might be plenty of reasons to capture peaceful creatures and the game mechanic should be available, but why people are so obsessed with doing it to elves boggles the mind. Did everyone's blonde macrobiotic mother lock them up in their rooms once?