Not legal advice by any stretch of the imagination, just some common sense thoughts. All relate to CME - Futures, no experience with US stocks.
Data remains the property of the exchange. What we think of as data providers eg eSignal, are actually re-sellers.
In my broker agreement;
Information Providers may have a proprietary interest in the Information that they provide, and you agree that you will not reproduce, retransmit, disseminate, sell, publish, broadcast, or otherwise exploit such Information in any medium except to the extent such Information is publicly available through no action on your part.
CME sells EOD data (daily updates) as follows;
1 Futures product - $120/mth
Any 5 futures products - $180/mth
All CME data incl Futures and Options - $1200/mth
CBOT and NYMEX monthly - same
CME Group ie all above - $1800/mth
1) Anytime someone collates something, they have rights to it. I can listen to the radio, write down all sports scores and post on my blog. If I scan the sports pages of The New York Times and post on my blog, they will come after me.
2) If someone is making money out of something, they will protect that. I can go to the ball-game, make detailed notes and write a game report on my blog. If I take a high quality video camera there and record the game and then stream from my blog, even if I do not get kicked out first, they will come after me later. They sell broadcast rights and they will protect that.
Think of an exchange like a stadium. Authorised people go there to engage in activities, information about which is controlled by the organiser. Joe public gets to go and watch, that's it. You can take photos for the album, blog about it, no problem. Encroach onto something that generates money for the organisers and you find them breathing down your neck.
If you want to know more without the cost of a lawyer (for now), visit the eSignal, Barcharts and IQFeed websites. Read the general package pages and see the declarations you have to make. Go into the signup pages and read all the notices and conditions and agreements. You can always cancel and exit at the point they ask for your credit card info.
That should give you an idea whether you want to pursue this further, and how.
Later on post a note about what you found, we can all learn from that.