Why do people feel the need to advertise it so loudly? I find it very odd.

The same reason why gay people come out and why women claim that they can do the same things men can. We are a minority group and we are denied basic things like divorce because the nation is Catholic. And like the OP said, there is stigma with being an atheist. Just the fact that we need to hide under "Roman Catholic pero di nagsisimba" is proof of that.

Depending on the atheist, the answer to your question could also be for the same reason why passionate Christians feel the need to convert non- religious people (or else these non-religious people will burn in hell.) Some atheists feel the need to save people from the grasps of religion. Sometimes, their method of arguing is wrong and extreme. However, most of the time, their arguments are justifiable when you take one look at the history of organized religion and how people were oppressed.

Personally tho, when I first realized as a naive teen that I was finally convinced I was an atheist, I tried to check with my peers how they feel about religion. Of course, it was "eureka" moment for me and I needed to explain my thoughts. Unfortunately, my excitement was only met with "You are going to hell" ironically from people who cheated their way through high school.

Today, I still don't understand why theist believe what they believe in, but as long as they're nice people and their belief in religion only has a positive impact on their lives, I'll just respect them, pray with them before meals, attend their sacraments and keep my lack of belief a secret. However, once they do absurd things like mention that gay people are going to hell or refuse to use modern medicine, I am going to be vocal.

Sorry. I tried to make this comment as short as possible. But it shouldn't be odd nor foolish to claim you are an atheist in the same way that a person can claim to be Christian.