The left protects immigrants (legal or not) because they are votes, and votes are power. So what if we have to tolerate a few terror attacks? The important things are protect the snail fish, allow abortion and GLBT rights.
While votes might be important, there is a far more important principle at stake. That principle is discrimination
To separate the sheep from the goats there must be discrimination. That is, a profile or profiles must be created from which decisions can be made based on various determinable and less than determinable facts.
Someone must discriminate to cull the objectionables.
To discriminate against those wanting to come in follows by extension that there can be discrimination within. When that happens, the black pressure for extended civil rights logically falls apart. It is ok to profile Travon and karisha