Yep, it's illegal. The test for trademark infringement isn't whether anyone's making money, it's whether there's a likelihood of consumer confusion, and here, it sounds like someone could think that your softball team is sponsored by or affiliated with the NBA team.
NBA teams can be very litigious, so don't be too surprised of they find out about this and someone on your team gets a nastygram from a lawyer representing the team.
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Ditto what you've already been told but in my opinion, this is such a minimal act of infringement that I would be shocked if the NBA either could learn of it or would do anything about it. I could be very wrong, in which case it is my opinion that they would ask you to stop and you would and that would be the end of it. Anything more than that would backfire on the NBA from a PR standpoint.
This answer is not legal advice nor should it be construed as such. I always attempt to provide factual information relevant to a question, but, in the end no attorney can properly advise a potential client based on the limited facts that can possibly be disclosed in a format such as this one.
It's not "our franchise", your team is not the Oklahoma City Thunder, that is the NBA's franchise and they make big bucks licensing their logo. This is illegal. It violates Federal law 15 USC 1114, and state law. If you could do it, why would anyone ever need to buy any officially licensed merchandise? Just swipe the logo and put it on a bunch of shirts and distribute them. Financially, that's actually stealing from the franchise not supporting it. Is that what you want to teach your kids? Will the Thunder sue you or have your shirts confiscated? Probably not - they like their fans, or maybe they see you as the main thief and just sue you since you would be the one behind it. The question is what will their copyright & trademark attorney, who is charged with stopping infringements of their copyrights and trademarks think? What will his boss think if he just sits by and lets people steal the logo? That the Thunder needs a different lawyer, maybe?
And don't think of switching to BOOMER SOONER either. The boys in Norman wouldn't have to allow that, although they tend to give fans a lot of slack, being that both their fans and team are a bunch of impetuous, wild & crazy kids generally with no money to pay any judgment.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
Yes. It is illegal.
The next time you watch an NBA game on television, right before the game begins, listen to the warning about unauthorized use of NBA intellectual property: "unauthorized use is strictly forbidden."
John W. Rooney, Ph.D., J.D.
USPTO-Registered Patent and Trademark Attorney
This posting is intended for general education and isn't "legal advice." It doesn't create or evidence an attorney-client relationship. You are encouraged to engage an attorney in the pertinent jurisdiction for confidential legal advice on matters of any importance. -John W. Rooney, Ph.D., J.D., http://johnwrooney.com