That force is the marketplace. A flat First Amendment standard is market driven. There is an inevitable race to go to the extreme, to catch attention through outrageousness, to push the envelope. The fewer the limitations on a medium, the better the ratings. Hard liquor manufacturers need to get Tequila and Kahlua onto television if they're get their market share up.
Broadcasters invoke the First Amendment to protect their profits.
In many ways it is a very positive thing that big business vigorously pursues First Amendment claims. They have the money and the interest to keep the pressure up, to keep speech free from intrusion. But the corporate view of what the First Amendment guarantees, and precisely what values it embodies, is not necessarily the same as how we as citizens might view the First Amendment's goals. The Founders conceived the First Amendment to make sure that government could not suppress speech. It has largely succeeded.