Ignoring the fact that selling something fraudulently is automatically bad, I can think of a few reasons. First of all, they can't make it identical. They can beat certain tests, but that's why it's a cat and mouse game.

Second, even if it was 100% identical, there are still reasons to support the "real" thing. If I buy fake syrup, I'm probably getting something made from an industrial monocrop like sugar beets or corn grown far away. If I buy honey from a local beekeeper, I'm investing in more trees/flowers/etc. in my own area. I'm also investing against the widespread use of pesticides harming our whole ecosystem.