You could also think about it this way:
1.) Baby (and/or their stuff) takes up a seat at the dining table, theater, and in the life boat.
2.) Baby likes to play in the Mickey ear!
3.) It sure helps having baby supplies like pack and plays, diaper genies, baby life jackets, changing tables, and seat slings. These all cost money.
4.) The baby is still a registered passenger and has to be accounted for, generating paperwork and administrative costs.
I can understand why someone would wonder about the charges, but like many have pointed out,
DCL does do quite a bit to accommodate babies on board.
I hope you have a great trip!