Wi-fi Log in page

I had the same problem and by calling Apple support we figured out this solution.

Keychain access> system> arrange by kind> delete all airport network passwords

Restart your computer and you will be able to connect to public wifi.

Hi there, adamstary.

I see that you're not getting a Wi-Fi login page when you're trying to connect with a public Wi-Fi network, and I'm glad to help.

Use safe mode to isolate issues with your Mac -- Restart your Mac in safe mode and if you get the login page. If it works while in safe mode, reboot normally and test again in your own user account. If not, continue to "How to test an issue in another user account on your Mac" below. 

How to test an issue in another user account on your Mac -- If the issue is also happening in safe mode, use this article to create and test a new administrative user account. This will demonstrate if the issue is isolated to your account, or if it's system wide.

Let me know how that goes, and take care!