Well, she started this the day she wrote the post, about the 2nd of November, and at that point (still, really) store clerks aren't even saying Happy Holidays. So I don't think they were _forbidden_ to say it, I think they thought she was weird for saying Merry Christmas three or so weeks before Thanksgiving. Apparently only one person even replied with Happy Holidays, and that was after she repeated Merry Christmas two times.
When I was growing up, I don't remember people saying "Merry Christmas" until very close to the actual day, to be honest. At department stores and tree lots, yes, but I remember teachers in school didn't say it until the last day of school before Christmas, and now that I'm thinking about it, I think it was more common to wish someone a Merry Christmas the last time you'd see them before the holiday, or at a Christmas party or play. It wasn't really used as the normal greeting all through December. Of course times change!
I'm thinking of varying "Merry Christmas" this year by learning the greeting in a few different languages. I think that would be very fun.