Well the logic is rather simple -- families make up a key customer group that fills hotels on weekends and other 'slow periods' and so hotels compete to attract them.

In this arena, the one thing that I find myself being a grouch about is the 'family' pricing for zoo/aquarium type memberships. At many places, my family of 3 (2 adults, 1 child) pays the same price for a 'family membership' as a family of 6 (or more) and it is a venue that is mostly oriented for the kids fun to begin with. And, then there is things like sibling discounts at child-care (which sometimes isn't much, but sometimes is steep enough to be off-putting) -- these are the types of things that make me feel like I am subsidizing other families paying full freight for my one kid.