I think it comes down to whether Mothers and Fathers days are private or public holidays.
If they are a private holiday, you celebrate with your parents/in-laws and children, a personal celebration of your choice. There is no confusion or ambiguity over whether someone is eligible to celebrate or not, or who you should be sending presents to, or giving good wishes to.
If they are public holidays, though, then you have the problem of seeing a woman on Mother's day and not knowing if she has children (and therefore should be wished a Happy Mother's day) or should be left in peace. So you can then decide that you will wish Happy Mother's day to any apparent female over the age of puberty, so you can't leave anyone out.
Personally, I find it rather silly when you end up congratulating a random stranger and giving them best wishes on a particular day because they may have at some point in their life, or may in the future, bear or adopt a child, based entirely on the fact that they are female (or male). Plus, for people who have made a conscious decision not to have children it can be kind of insulting, and for people who want children but can't have them it can be pretty painful.