I had never encountered the term "sea squirt tenure" before so I went Googling and found this:

From Consciousness Explained by Daniel C. Dennett (Little, Brown and Company, Boston, 1991):

The juvenile sea squirt wanders through the sea searching for a suitable rock or hunk of coral to cling to and make its home for life. For this task it has a rudimentary nervous system. When it finds its spot and takes root, it doesn't need its brain any more so it eats it. It's rather like getting tenure.

That's pretty harsh, but it probably sums up the Republican view of why laws that protect employees are a bad idea.