thought of it from Disney's point of view - if they gave out a free day or a $70 gift card for wedding anniversaries - they'd have to give out two. That's $140 worth of giveaways for each couple. And then they'd get complaints that people who aren't married or are too young to be married or are divorced or widowed or whatever don't have as much of a shot at the promotion as married couples do.
And then what other celebrations should they give away free stuff for? Graduations? Family reunions? Engagements?
Everyone has a birthday, so everyone has the option to go or not to go for that day.
As has been noted - with this promotion you can be absolutely assured of getting a free gift of some sort if you register and show up on your birthday. With the current promotion, you're likely to get jack squat.