That is correct, HTML files are automatically upgraded to ZIP files with all linked resources etc. contained within.
Is there something you prefer about disassociated HTML files? (I believe you can disable the HTML Input plugin to stop that automatic conversion.)

Does your device actually allow you to read HTML files?
I think what you probably want to do is convert the ZIP container to an EPUB.
P.S. Welcome to MobileRead, and welcome to the Association of calibre Lovers.