Using Unity is different from Unity the company directly working with defense contracts. I can see people taking moral issue with that and not everyone working at Unity is a US citizen and even if you are you don't get a choice in how your taxes are used. Whereas this agreement is voluntary support. From a user perspective it's also at odds with the public image from the history of the company even if it's been a historically good platform for other companies to develop defense simulations and training tools with.