Of course you're allowed to criticize the religion. Generally people aren't called Christianophobes because of a couple factors:
1) There isn't nearly as much irrational criticism of Christianity. Most people in the west are more familiar with the religion, so they criticize it more accurately.
2) The uncommon times there is similarly irrational criticism of Christianity, it doesn't lead to the same widespread animus as there is with Islam, and we don't see the same sort of reaction against Christianity as we do against Islam. The dynamic has a lot to do with the sizes of the groups. In the west, Muslims are the small minority, whereas Christians are the majority.
Almost. Yet again, it's a word used to deter irrational criticism of Islam.