Question regarding USB and LPT1 ports

We installed an HP USB scanner to one of our PCs in purchasing. They currently use a DOS based program called ESI to complete thier ordering for our facility. This software utilizes LPT1 as its default printer port. after installing the scanner the program would no longer print. I didnt think USB would interrupt LPT1 ports? We fixed the problem by tricking the program to use LPT2 but I would like to kill my curiosity. Thanks for any help in clearing this up for me.