As disappointing as this is, I can see how this makes sense from the standpoint of being the people who actually have to ENFORCE something like this. As much as it sucks for those offering customs, it is such a gray area of trading as it is. For TNT to investigate this, they'd need to look at neomails and judge whether the trade was confirmed, check logs to see when the potion/paint brush was obtained, when exactly the pet was painted or morphed, if the other user had attempted to back out before that happened, and then decide the appropriate action. While I've heard of cases where people back out AFTER a custom has been created, I've also heard of the custom-creator backing out after making a name the other person liked, and even a trade where a person said they were CONSIDERING the custom offer, when the person made the custom and then told them they had to trade or they would be frozen. The custom scenario is so tricky, and while it sucks to be the person left holding a pet, I also don't know if I'm comfortable with someone being obligated to go through with a trade when they want out. On the trading post a trade is not final until it is done, and some people arrange trades on the pound board MONTHS in advance. So much can happen in that amount of time.
Eventually the buck has to stop somewhere, and as much as it sucks, I feel that the risk should be with the person making the custom. A trade isn't final until the pets have been exchanged, and really I think it's kind of always been this way-- I've never seen anyone actually frozen for backing out. Now those who make customs are aware of the risk they are taking, which I think is better. Even if the other user gets frozen, you are REALLY not going through with the trade then, so you are still taking the fall even under the old rules.
Also I would HATE to see Neopets traded for money, just because I can only imagine the horrifying pricetags attached to certain UCs and battle pets, and since there isn't any way to hide your pets it would be like drawing a big target for scammers on your forehead. It's bad enough now when they can only be traded for other Neopets, but at least it keeps the trading "price" somewhat in control.
Plus I just don't know how I would feel if I found out I was sitting on a 30 million neopoint pet.