In what way does that page "promote proprietary software"? Its entire purpose is to help improve a piece of *free* software (Zope); the only mentions of Launchpad are the home page link (the logo at the top), and a "Help us improve Launchpad" link at the very bottom.
The report contains valuable information, including a patch, about a bug in an application I help maintain. Rejecting a link to this page because LP happens to be closed would be a ludicrously irresponsible policy. Making the report and patch inaccessible to the maintainers of a putative upstream application which applied such a policy would be a disservice both to the maintainers and to the users of the software.
Note that I *do* wish LP were opened up, as I believe its value would be increased. I *don't* think rejecting a link to a bugreport in LP would be sane: it would actively harm the free software ecosystem.