Because that pretty much looks like a moving goalpost.
Interesting that the complexity of the rules seems to undermine even the simplest of things. I think that the amount of time needed to prepare for this would definitely be increased by first having to create a "friggin'" dragon from the books, then having to give him "Detect Magic", and in the end it doesn't even work how you wanted.
Must be frustrating.
What's funny is that even moving the goalposts, the ball still isn't going through.
It still doesn't tell the dragon that the party is coated in magic; it tells him that the party has at least one magic item. That's it. It could be a crappy 1 dart for all he knows.