Well if you look at it, unless you use "text" for "surfing" like in this app, it actually does not cost much to the providers. You can not type all day, can you?
So by "unlimited" text all they are saying is that you will tire yourself texting say about X SMS and thats it per day.
1000 SMS * 160 chars * 2 bytes each (arbitrary) = 320000 bytes. Thats just 320KB. Thats cheap isn't it? Can you type more than that per day?