Existe um tópico no debate da Apple sobre este problema: Problemas com o hotspot de Wi-Fi público .
Uma sugestão:
Nothing in your MBP is seriously miss-configured. Most times it's the service. What I do is to create a new location for every public hotspot I visit. Outline the preferences along with ISP (network names) in my Preferred networks list.
If a hotspot reacts like the one's that are giving you issues I manually type the hotspots sign-in page in the url address in my browser. Once there I can bookmark that page for the next time it gets buggy.