Then their whole "We care about our City Elf brethren" line that those Dalish who don't see them as traitors or subelfs say is a lie. If they care, they wouldn't do what the humans did and force them to either adhere to their culture or fend for themselves.
The Dalish are surviving in an environment where their religion was criminalized by the Chantry, where human lords and lynch mobs drive them off land when they stay too long, and where templars pursue them. It's not as though the Dalish are living an idyllic existence, they live a life of hardship, but one that enables them to live free from human rule.
A Dalish elf can enter an Alienage simple enough, and elves can leave just as easily. We've seen both happen ingame.
I'd say that Varric's connections to the templars (like his friendship with Ser Thrask) helped out with Marethari entering the Alienage with ease, given that there's little doubt that she would have been apprehended by the templars otherwise. Not to mention that the law simply isn't on the side of the elves if there's trouble; in Ferelden, for example, it's illegal to kill a human in defense of an elf.