If religious people can pretty much advertise wherever they want, then atheists should be allowed to as well. To be honest I would much rather see an advertisement from an atheist, than the same old tired religious advertisement. I really get tired of seeing them. I think an atheist advertisement would be entirely appropriate!
Atheism makes people uncomfortable, but that is not a crime. They have the right to express their opinion and their views as long as they don't criticize the views of others. If they would like to advertise they have every right to. Other official churches do it all the time. Let them be.
Advertisements have huge influence on teens. Therefore these shouldn't inspire them to become atheists. Religions motivate you and make you strong. If someone is meant to become an atheist he will, it's not a hard thing to understand. If you are not really an atheist but drifted by these ads to become one, that's a shame.