You are not forced to log in, you can always choose to use the Full Version as it was before without logging in, if you so prefer. Just mind that in doing so you will lose most of the features added with the latest update/s.
For the reason why, it's simple: Minimal Counter needs an account to sync your Counters to all your devices. I tried to make the login process as painless as possible (You can even login with your Facebook or Twitter account) if you don't want to remember yet another password.