I would worry that a young Earth creationist might make a poor designer. Someone who might ignore the evidence of a design error in favor of some other interpretation not supported by evidence.
Why did the deity put the fossils there? In order to confuse us? In order to challenge the faithful? It defies reason. You can only put it as a mystery of faith. But it needn't be.
I always thought the most plausible explanation is one developed Terry Pratchett in "Strata":
If you are going to create a planet (or universe) from scratch to be the home of an intelligent species, including a realistic history is critical to give that species sufficient historical context to learn about how things work.
Understanding evolution and biology is a lot harder if you have no fossil record. Understanding plate tectonics is a lot harder if you don't have a consitent geologix record. Understanding physics is a lot harder if you don't have a light coming in from events that predates the creation of the universe.
Thus if you grant that the universe was created 7000 years ago with a huge, detailed and consistent history, the clear implication to me is that we are intended to study that history to understand the universe in which we exist.
This reminds me of being a teen and getting my mind blown when a friend of mine said, "You can't prove that you didn't just start existing right now. Who's to say that all of the memories of your entire life were all implanted in your head to make you think the universe didn't start right now."
In my first HW design job out of college, I was very surprised when a senior engineer made that exact claim -- that God planted dinosaur fossils in the ground. I knew he was very religious and I could tell he was quite serious, but it was long ago and I had never heard of Young Earth Creationism, so I was shocked into silence. (Also, he was my manager and I decided I couldn't afford to argue.)
My dad tried to say the same thing. I countered with that in the Book of Job the devil couldn't do anything to Job without God's consent, so that would mean that God was trying to intentionally fool people with no way to prove what we can see with our own eyes wrong.
He changed the subject and never brought it back up to me again.
Why did the deity put the fossils there? In order to confuse us? In order to challenge the faithful? It defies reason. You can only put it as a mystery of faith. But it needn't be.