According to this article, that's a risk that employers run from condoning an office pool on the NCAA's annual basketball tournament. But do employers realistically run the risk of a discrimination or retaliation suit because some employee can't come up with $5 to enter the pool, as the article suggests?
I've been at employment law for more than a couple of decades now, and I have never heard of an office pool causing that kind of problem. If anyone knows of a company that was sued on such a theory, I'd like to hear about it.
In the meantime, just use a little common sense and let your employees have some fun.