Why is kaffir lime plants banned in the USA?

it has to due to a fungus they are known to carry, which could potentially kill all the citrus plants in the us

Kaffir Lime Plant For Sale

It's been a while since your question was answered but I have to add my two cents..:) I lived and traveled in Thailand since 1970 and the wondrous smell on fresh kaffir lime leaves brings back an ocean of memories. The only problem I have is the leaf itself has the consistency and taste of hedge clippings and I never enjoyed the process of fishing them out of my bowl.. I recently purchased some essential oil online from a Thailand-based distributor. Indonesian sources are usually made from dried leaves which makes the taste and smell a bit different. The oil is very potent and very little, sometimes less than a drop, is needed for a true authentic taste...and no more fishin'.

I don't know why this specific tree was banned, but I do know that most plants are not allowed to be shipped into the USA grown in soil. IE a potted plant, a tree/ shrub dug up and shipped. the USDA is afraid of who knows what could be in this soil. Fungus, viruses, bugs, etc. Also there are a few major problems with citrus trees these days. 1 is citrus canker, and the other is citrus greening. the govt wants to prevent the spread of these 2 diseases. thank you