My mom buys IAP all the time. My brother would do the same thing if he had a credit card on file on his account too. I took mine off the first time I saw him do it, and I don't let my mom use my card anymore because she was just racking them up.
Most of the people who do it have no value of money. If they weren't spending the little they have on this stuff, they'd find somewhere else to throw it away on. It's not like G5 Games, Big Fish, and all those other publishers that have like hundreds of games haven't been doing the same thing over the years.
It's a business model, and it works. I don't agree with it, but I'm already doing something about it by making sure I'm not wasting my time with them myself. Everyone in my house already thinks I'm weird, lecturing them on this stuff won't do anything except make me walk out of the room very angry for even trying.
Sounds like you are doing totally the right thing, & I applaud you. When even your Mom is falling into the trap, it takes guts to stand up & not let money that could otherwise go to better things be used up.