Why do we have to register to view profiles?

Because does the whole world really need to able to see user's profiles? Some people have links to blogs etc. on their profiles. While users are only permitted to include links to blogs etc. that are not personally identifying, those details do not need to be available to the entire internet.

Needing to sign up/log in to view profiles has seemed to be fairly standard for most forums I've come across.

It would bring more visitors to those blogs, and it's not like there's anything private in those profiles.