Well, that's simple enough to understand. Forge World's resin production turnaround is really quick - look at the lead time between the first week of Knight sales and their upgrade kits and variants. Even with Horus Heresy, the specifics of book releases and specific models aren't as detailed up front as GW.
Compare that to GW, where there's about a 2 year lead time, if not more. Their design processes have been pared to efficiency, but at the cost of the ability to react quickly to change.
Look at how quickly the relatively grav gun release leak from the 6th edition Marine Codex resulted in competitors getting early to market with alternatives. That will have directly cost GW money in Tactical Squad sales.
It would appear that early leaks actively cost GW money in terms of lost sales. I also can't see that it would add to the number of kits people actually purchase. The internet rumour community is pretty small all told, and doesn't account for a large portion of GW's sales. Sure, it's exciting to know what's going on and nice to be able to plan our spending - but what serious change in spending habits would knowing things in advance actually cause? Likely a reduction in spending as people saved up for a forthcoming release.
With the law in the US arguably claiming that artwork doesn't protect a sculpted miniature's design, GW can't do anything but have a tiny, tiny lead time. There is a definite risk for them that an early leak of a design could mean someone else gets to market first and claims the copyright over that design, causing them to need to scrap a section of IP and throw out a lot of work.
Maybe the court case wouldn't go that way.
But it's better to just not have another court case.
There's no money in letting leaks happen or previewing things far in advance. There's a ton of legal risk in it.
Once you look at it from GW's point of view, there's no other course of action to take.