Free wifi is now common, but in my experience well-performing and reliable wifi is not, whether free or pay. I've only encountered the two tier (free or pay) situation once, so I don't think that is common. At least not yet. Like many others here, I carried at cellular modem to handle my internet access requirements. I did that rather than using my phone because it stays at the site when I walk away with phone in my pocket, enabling my wife to continue using her tablet, e-reader, etc.
It's not just campgrounds - my experience with free wifi in all sorts of businesses is mostly dismal, whether store or restaurant, large or small. Typically they are just allowing customers top piggy-back on the business' own internet connection and the coverage and the pipe to the internet simply isn't geared for the extra load and space. It lets them advertise that they have "free wifi" without spending much money on it.
Sadly, the fee wifi services aren't much better. The park often still lacks good high-speed access to the internet, so the paid service may still be slow or erratic. And nobody in the park can fix problems with the fee service, they can only call the provider who may not even respond for a few days.
