Actually, people haven't been saying the NFL tweets (or lack thereof) are in any way race related, but thanks for playing, though.
However it does suggest that past character issues or playing ability may not be the problem, while perhaps exercising free speech is.
Johnny Manziel was an admitted drug/alcohol addict and has committed domestic abuse. His career numbers are 7 TDs/7 INTs and has reportedly 12 teams looking at him. Meanwhile Kaepernick, comparatively has a pretty clean background record, has career numbers of 72 TDs/30 INTs, and took his team to the Super Bowl with 0 teams expressing any interest.