Do you think people in other countries who have other majority religions should say "Happy Holidays" so as not to offend people who might not share their religion?
I think American businesses say happy holidays because they want more than just Christians spending lots of money on gifts. I personsonally don't give a shit either way. I prefer the cashier to just ring me up and not engage in banal greetings. I think the change in this speech pattern came from retailers and not because people were offended with merry Christmas.
I don't care for banal greetings either. I'm with you there. I want to pay and get out.
However, in many workplaces I've worked at, there seems to have been a pall brought on by HR at least discouraging saying Merry Christmas and opting for Happy Holidays. I've had managers self-correct themselves and correct others as they spoke -probably out of fear of offending people who take the greeting too literally [i.e. celebrating Christ Mass].