I believe the concern is 2 fold. You can sell trained pets, and there's the possibility of tamers afking in dungeons, earning rewards while afk, or scripting and looting corpses that the pet kills while training afk.
It's a valid concern. But I'm not sure this was happening to begin with, and I truly do not believe it's something that will ever be a major problem on our shard.
The selling of trained pets: Really, there is no market on our shard. We do not have the player base to support this market. If anything, 1 or 2 pets here or there might be sold. Over the course of months. There's only a few of us that primarily play tamers. If we were making mad bank from selling trained pets, more of you would do it.
The farming items and points while afk: Yeah, this is a tough one. I get the fear of the possibility of abuse. And I can only speculate, because I'm a player with no behind the scenes server data, but I don't think this was something that was commonly occurring. I think this is more of a fear of potential abuse, especially with the removal of bonded pets losing loyalty. It's a perfectly rational fear. And if we had a large player base I think it would be a required rule. But given our current player numbers, I don't think this is something that will occur.
I've seen mentioned about pet training within a custom house with this design in mind. I've read forums of people doing it. And while it's a fantastic idea, it's hard for a new player due to the huge cost associated with Heritage homes.
Also, someone would have to correct me about this, but in utilizing multiple account to train off of each others pets, or even using a single account, and having your two pets skill off of one another, would you not need to be guilded? I remember being guilded being a key part of training without monsters a long time ago. I'm not sure if the mechanics are still the same or not. I bring this up because Heritage also has a huge upfront price of 250k to form a guild. I believe OSI is only 25k. This would also be hard for a player new to the server.