This M.Y. answer says that if a Chattan is in shul, tachanun is skipped.
Art Scroll and Birnbaum Siddurim say that Tachanun is skipped in the house of a mourner. However, if the mourner is in shul, tachanun is still said.
What is the difference between a mourner being in shul vs. a chattan being there that causes the entire shul to skip / not skip tachanun? WHy shouldn't they have the same rule for both?
It seems that there is already a precedence not to say Tachanun in the presence of a mourner, but its restricted to when you are praying in his home. But when he comes to shul, his presence has no effect on tachanun.