I have no idea why a fence contractor would say that unless they don't have access to the product and are giving you a story. Presumably you have a land line...call them and ask. And, in any case, you should be getting estimates from more than one fence contractor.

I have posted about my Black Locust experience. Make sure you are purchasing from a reliable dealer!!!! Not Black Locust Lumber Global. . I wrote extensive reviews with pictures on Google. The wood being harvested by some is a fast growing weaker variety harvested in foreign countries. it is definitely possible and legal to purchase it in the US and grown in the US.

There is a large distributor in Jersey area. Do your homework and make sure of your sources. c

https://www.houzz.com/discussions/5957580/ideas-for-railing-with-a-black-locust-deck