The Concorde Agreement covers the financial affairs of F1. It states how the income is split between the FIA, the F1 teams as well as stipulating the prize money for the teams and drivers*.
Sponsorship is essential to F1 (as it is to most sports) and without it F1 would die or at least exist in a form much smaller and very different to how it is now.
The sponsors see the cars as moving advertisements and want their names on as many TV screens as possible. If the viewing figures fall because the sport is only shown on pay for TV the sponsors wont be willing to spend as much money therefore the teams lose income.
Bernie is only interested in making money but he realises that just taking the biggest offers is not always for the best in the long term.
The F1 teams were fully against Sky being the sole F1 broadcaster in the UK because they know sponsorship will fall.
* »
en.wikipedia.org/wiki/Co ··· greement is a better description than I will ever be able to give.
Edit: I've just found
this article on the BBC.
quote:
The Concorde Agreement is secret - so secret that the teams are not even allowed to retain their own copy - but it is known that it contains a clause which dictates that, in major territories, F1 has to be broadcast on free-to-air television.
The reason for this is that free-to-air TV has much bigger audiences than pay-per-view, and the bigger the audience, the greater the chance of bigger sponsorship deals and therefore financial security and, for the teams, on-track success.
Take F1 off free-to-air TV and the audience would shrink dramatically. The smaller the audience, the less keen sponsors are to be involved, and the less money those that are involved would pay to the teams and Ecclestone.
So perhaps the US doesn't have a big enough fan base for the Concorde Agreement to apply but F1 and Bernie still want to break into the US market because they know that it is worth millions. By limiting the viewing audience they are making it harder for themselves to achieve the break through.