He is lending visible support to an issue, that is quite important, much like the press conference held by Muhammad Ali, Bill Russell, Jim Brown, Kareem Abdul-Jabbar and Willie Davis in opposition to the Vietnam War. Given how corporatized sports has become, it's a wonder anyone is making their voice heard, especially considering how the athletes in the WNBA were threatened. Moreover, how do you know he is not doing more? You've just noticed this action by him, but he's spoken out on issues before.
A movement requires more than a Mother Teresa or someone "going green," it requires policy changes. You complain about Al Gore traveling around the world, but his goal is policy change, which will do more to stem the issues of climate change than not flying in a plane. The goal is not no pollution, it is to change the overall level of pollution, saying his world travel is causing significant pollution is a specious argument. People working on climate change know that encouraging people to use energy saving bulbs in meaningless when they derive all their energy from coal.
Someone mentioned Michael Jordan, but I don't see how he really took a stand on anything. He gave money to a police organization that studies issues and makes policy suggestions to police departments; I don't think the problem requires more study, we know the issue and we know the solutions that work. Likewise, he gave money to the NAACP Legal Defense Fund, which is not working on the issue of police brutality, they're working on an array of issues, but his money would have been better used directed at a group directly working on the problem, like a local chapter of Black Lives Matter. The reality is Jordan gave a corporate statement that doesn't really address the issue and gave money to organizations not directly working on the problem. While these organizations both do good work, and it's good to see someone support their endeavors, let's not pretend as if his actions has any relation to stemming issues of police brutality.
But I agree, more action solidifies your standing on speaking out. But sometimes just standing up is enough, Tommie Smith, Peter Norman and John Carlos were instrumental to movements on human rights. While I don't think his actions rise to the level of Tommie Smith et al., it is not meaningless. I would argue that ad hominem attacks saying an athlete is washed up shows a certain lack of maturity because that is simply lashing out at someone you disagree with.