What prevents somebody from purchasing a data stream and then dumping all the data online somewhere, allowing others access without paying?
Short Answer: Nothing
Long Answer: It is exactly the same with music and TV series. They also get re-uploaded illegally in the darknet.
This problem is, however, unrelated to IOTA and as ben75 already said in his comment
more a 'licensing' problem.
As a result of this data publishers may increase their prices, so rebroadcasters have to pay more. As a result, they will also have to charge a fee, to reach break even.
However, using "rebroadcasted" data is everything but save! The "rebroadcaster" could easily change the decrypted data and republish "fake news" instead. Therefore I do not think this will become a major issue in the future as most users will be advised to stay with the "official" sources