Well it is an IT issue, as their website can't differentiate between a child and an extra adult in the room, even though they make you put the age in.

Say airlines for example, they make it very clear. They have two different categories, an infant who doesn't require a seat, and a child who does require a seat. It wouldn't be too hard for the website to determine that guests under the age of 2 don't need a bed. And if they do, the parents will specifically book a room with more than one bed.

Also, saying it's not specific to Marriott is irrelevant, as we are discussing Marriott here, not other hotel brands.