Even religious people have a concept of "bad religion" -- the religion of their enemies. What they worship is not some abstract concept of religion in general, but one specific religion. Other specific religions are wrong.

Saying "atheism is a religion" is reducing a new enemy to a subtype of the old enemy they already have millenia of experience fighting against. It's not suggesting that atheism is equivalent to e.g. Christianity (something high status), but rather that it is equivalent to Baal worship (something low status).