Because when you pay for a pattern or magazine you shouldn't have to hunt to find out if there are mistakes. They have everyone's email that has subscribed. They should be notifying people. Or better yet, like others have mentioned, use pattern testers before releasing any. It's not fair to the first few that try a pattern to have to be unpaid guinea pigs and be the ones to discover and report errors.
No matter how easy it is to find out about the errors, the bottom line is that there shouldn't be so many in the first place. On a free publication, sure. When you pay for it, absolutely not.