Atheism is no more a religion than someone who is enthusiastically devoted to Coke over Pepsi!
And some theologians would consider over enthusiastic devotion to a soft drink to be idolatry. they would have a point.
You are absolutely correct about the fact that this is evangelically motivated. I don't think such a sticker would be necessary; EXCEPT that scientific textbooks in schools are usually terrible, and are often written from the atheist standpoint; with dogmatic assertions about how evolution works that often present bad argumentation even from a scientific standpoint. I've been caught out assaulting outmoded theories, theories that were presented to me as being FACT and I was tested as to my acceptance of these theories.
What's the harm in calling it a theory? It makes the religious types feel safer, has no bearing on day to day life, and at least allows for the remote possibility that our concept of evolution might need substantial revision someday. It is more honest to say that most scientists accept evolution as a fact, rather than test a student on a true / false loyalty oath that will probably prove wrong in time.
For instance, I wrote down True, after some agonizing. I decided that spitting up what the teacher wanted was more productive than defending my vision of God. So I wrote down that evolution, the theory that was presented to me in class, was a fact. However, I learned later that almost everything that I had learned, and accepted as a fact, in that class, was rendered irrelevant. From what I recall the theory was that viruses and radiation create most DNA mutations, that cellular life had originated in hydrothermal beds in the sea, and that evolution was a slow, steady process. (All these theories have been discounted, since).
I think it was immoral to make me say that I accepted all this as fact, particularly when it contradicted my religious beliefs at the time, in a class I was forced to take by the State. In a sense, the State was forcing me to renounce my faith. Which is a disturbing exercise. Now if the class were optional, or in a private school, or had even acknowledged that the dogma it was teaching me might be disproven by fresh evidence someday... well... I would have felt better about it. That's all the sticker is doing. Making some kids feel better about studying evolution.