I don't think they limit you, the speeds are just so slow that you wouldn't want to web-surf with them. Its similar to the ways a lot of hotels parce up their internet services (often with "email", "web surfing", and "video streaming" flavors) . I think listing email & facebooking are just attempts to provide an example of a traditionally low bandwidth use. They could also say "reading mostly text based web sites" but that's a little wordy. It would be nice if they just gave the speeds, but I doubt they want to make any sort of commitment in that direction.

On a recent trip on an A319 with wi-fi, I used the slower option between BOS & ORD and was able to use email and the web just fine. It was a bit slower and I could see how it would be frustrating for anyone wanting to entertain themselves via web.

As an aside, many of my emails are >5MB with attachments. I can imagine the slower version wouldn't make that email experience very productive.

As a second aside, I think it's interesting they don't provide any examples of potential applications for the faster speed. They just basically say "it's faster" with three different bullet points. I assume they don't want to give anyone ideas of how to suck up bandwidth. They might as well just label the two options "on your own dime" & "expense account" since I am guessing that is the driving factor in most peoples decisions as to what option to choose.