Same place he pulls all his other cranky ideas about darn kids not playing pinball on location and diluting his precious. Just compliment the onion on his belt and don't make eye contact is my advice.
I'll also say that toppers, side art, shooter rods, basically anything Stern is offering have been commercial mainstays since before Pinside existed, let alone the modding community. They're hardly "stealing" anything. Furthermore it's highly unlikely they're going to go back and offer anything from their back catalog, so we're talking brand new games only.
If you're doing pinball mods as a full time business and trying to chase only the newest stuff then yeah, they're competition. They have the licenses and aren't as likely to look the other way as in the past. So it's strong competition. Just means you either have to be better and smarter and faster and cheaper (or some combination thereof) or instead focus on the decades of awesome existing games that people love.
I don't really see it as an issue myself. I will say as someone who dances around the grey areas of things (I do original art, but my projects are too small time to be able to be licensed) I wouldn't personally try and compete with Stern. Just seems like asking for more trouble than it's worth. But this isn't a job for me, I do it because I enjoy it, and it makes people happy, and the money I make from it just goes back into pinball. So Terry or Stern or the guy trying to sell his Fathom (hey, who's doing that? Let me know!) end up with any profits in the end.