Many "free" wifi services, such as restaurants, coffee shops, hotels, require you to advance through an agreement prior to full access being required.   These services will intercept browser traffic and redirect you to their registration site before you can gain access to other sites, including email, or other cloud-based services.

If you have your iPhone set to Automatically Join Networks it will connect to the local router, but not further, until the registration is complete.  The issue is that until you browse, you will never be prompted to complete this registration and expected internet connections will not be available.