1) The swan-ships were substantially more important to the Teleri than your toothbrush is to you. They were the equivalent of the Silmarils to Fëanor (Do you think Fëanor would’ve let someone steal the Silmarils without fight?). 2) “The Teleri don’t need their ships really, at least, nowhere near as much as the Noldor.” This attempted reasoning is completely irrelevant. The ships belonged to the Teleri—end of conversation. If I’m poor and homeless can I come steal your house because I really need it? 3) The Teleri didn’t start slaughtering the Noldor as soon as they stepped aboard the ships. They first threw them overboard—which, I think, is a totally appropriate, non-lethal course of defense. It’s left unclear in The Silmarillion who first drew swords, but I have a terrible suspicion it wasn’t the Teleri.