They have made an adaptation. It is not the first one and it would probably won't be the last.

Changes on adaptations happen all the time. Take Stephen King. Many of his books have been adapted and the shows have changes. No one care and Stephen King doesn't care either.

Gaiman's comics and some books have been adapted. There have been some changes and he is OK with that.

Then you have the classics: also adapted. Like Shakespeare with some versions more terrible than others (Except Keith Branagh that uses the original script).

My point it, you are allowed to not like it, but versions of original books and scripts are normal and would keep happening. Some would be good and some won't. But that is OK and cames with creation. I for one enjoyed the show even if it is not completelly true to the the book.