That's a little absurd. You would think they would have kept the two separate. I wonder if they only allowed it or did it that way because it's getting more more sales of d2r from bots transferring over.
Quite simple to understand though
Contrary to the conspiracy theories, blizzard makes no money at all off the secondary market. Their only profit is when people buy the games, and that's the only point they have to care about.
There is a profit motive to encourage people who only have LoD to buy a copy of D2r+RotW and be able to convert their characters seemlessly, who might be turned off if they had to abandon them for a fresh ladder start
It might not be a big segment of the market, but if I offered you $1000 right now for the difference between 'lod bots allowed on rotw', would you refuse it to keep them off? Now the 1-3 people actually working on classic games team.