This is correct. It's a conscious effort by the user to opt-in to updates on the thread. As the seller, simply reply to the thread with whatever you'd like them to see.
I also agree that users don't know of/use the bookmark function often, and in no way does bookmarking a listing tell me, the user, that I will get mass PM'd about the listing. I am familiar with the eBay feature you're referring to, and I don't appreciate being spammed because I watched an item, either. That kind of function is on the "sneaky" side for me, and I don't think we'll be implementing it.
Subscribing is what you are asking for, and the function already exists.
