Huh so it's pulled an FMA then. I hate it when studios jump the gun on the author in order to keep earning money...
Practicalities though. None of the kids would be kids if the waited for GRRM (they are already old for their parts). TV-only fans would be super pissed if they had to wait. The core creative team have centered their lives over this for over a decade now, would makes sense they want to do something else in their lives too. They've been on an annual schedule for years now, would be a big investment to put it to stop and the then start up again after a long (several years) break. They are already taking some more production time to finish the series with glory. I'm pretty sure some of this time is taken to consult with GRRM and to let the shows own writers figure out the ending they wanna do.
It was kinda annoying to me at first too, but now I'm more of the mind, that the books and the show are two different entities, and I'm pretty certain that even if the show "spoils" some central points, it's not gonna ruin the reading experience of the books, and the the books are gonna go down a different over all route.