Besides being a waste of energy, it's also free promotion for atheism.
It's hard for some people to believe, but "organized atheism" - orgs like the AHA, FFRF, AU, GAPAC, etc - is small. Tiny, even.
If it weren't for coverage, mostly generated by Christians, they would largely go unnoticed.
It's not free; they paid for it. They have just as much a right to buy ads as you do.