DaasYochid, by wearing a WCS shirt or any other t-shirt, whether or not you got it for free, you are, like it or not, expressing an approval for that organization. That’s just the facts.
You may mean differently, and that’s very nice in your own thoughts, but, in the real world, wearing a logo implies some recognition/support of what that logo represents.
With a Sports Team, for example, it could mean you enjoy watching their games or attending their games in person. It could also mean you consider them to be role models or more; but it doesn’t necessarily mean that.
There is no possibility of kosher endorsement of the IDF. The best you can say is that, once the Zionists made the mess they did, the IDF is now needed to deal with that mess. Arguably. But that’s not why one wears a t-shirt or sweat-shirt, just like you wouldn’t wear a sweatshirt with a toilet plunger (except as an an inappropriate joke) despite its utility, so the obvious implication of wearing an IDF sweatshirt is more than that, one of endorsement, which is inappropriate on a number of levels.