This is so common nowadays that for many sites I have the direct login page bookmarked. It indirectly implies that once you sign-up, the company stops caring about you. At-least for me.
It implies that because it’s literally true. Not a complete loss of care, but it makes sense to hide the sign in button because users who are already signed up are already invested, and less likely to abandon the service. The front pages main job is to grow the company by attracting new users, and a sign in button for users who aren’t going anywhere anyways gets in the way of that.
While that may be the case (I'd also argue that the front page should be a welcoming place for existing users), showing me that you value your existing users is a great marketing move.
It's otherwise very hard to convey that you care about existing customers so this seems like a no-brainer.