Colin Kaepernick is better than Thad Lewis, but that is irrelevant. Lewis was signed by the Baltimore Ravens because he was better than Dustin Vaughn. He wasn’t signed to be an insurance policy to Joe Flacco. He wasn’t signed to replace Ryan Mallett. Kaepernick may or may not be being black balled from the NFL, however this has nothing to do with him.
In training camp you need several quarterbacks. Every team employs quarterbacks that they probably won’t keep on the active roster. Lewis is an experienced player who can step in and get some work done behind Mallett.