Derek, I have not found this to be true with the default installation [of oscommerce]. session ID is included in the URL? I think static HTML files may have broader compatibility with the smaller niche engines
There's a toggle option in the osC admin to turn off Session strings (Can be found in Config > Sessions > prevent spider sessions). Additionally, there are contributions (read: plugins) for search engine friendly URL (imaginatively titled SEF links or suchlike) re-writing.