It's pretty easy to figure out, if we want to be honest. First, most American companies are headed by decidedly left leaning corporate leaders. And nobody loves ending freedoms above the waistline more than the modern Left.
Second, most in our society support sex deviations out of convenience, since a growing number of Americans prefer a hedonistic society over a religious one. This is something corporate America prefers as well, since hedonistic Americans make better consumers than ones dedicated to a religion of poverty and moderation.
Which leads to third reason, in which we concede that most Americans are not religious, or if they are religious, they place their religion somewhere in the bottom half of the top ten list of priorities, if there.
Fourth, Americans have been indoctrinated against traditional Christian values for almost half a century. Younger Americans (who many of these companies cater to), have been raised to believe that the Western Tradition and its Christian heritage are the true evils of history, and all who challenge these value systems and Christian morality are by default the good guys. See the comment by reader 'Tom Bradshaw' for an example of the hatred and contempt for any who desire the right to believe in traditional religious morality.
Finally, Corporate America is about the bottom line, and if the bottom line is buttressed by a majority of people who want to end other people's freedoms, then that is who they will cater to. Since the willingness to fight has long left the majority of Christians, and those who still desire to resist are increasingly piled on by fellow believers both to the left of center, as well as those who would try to appear as the voices of reason, there just isn't a sound, financial reason based on demographics to do other than throw their corporate logos decidedly on the side of oppression and retribution for failing to be liberal. After all, most of these companies also have plenty of revenue outside of the US, so if there was to be a 'what comes around, goes around' backlash down the road, they wouldn't be the ones to pay the price anyway. They would just move physically to be where their bank accounts and tax shelters have been all along.