I have a strong interest in this as cycle.travel carries out more detailed analysis of access tags than, I believe, any other end-user website.
I agree that it isn't reasonable for worldwide sites to parse E+W-specific values of the designation= tag (to my knowledge, cycle.travel is the only one that does).
However, it is [widely understood and accepted](http://wiki.osm.org/wiki/OSM_tags_for_routing/Access-Restrictions) that
1. highway= tags imply default access values;
2. these access values may vary from country to country.
This is why we moved away from tagging horse=yes on every single road back in 2006 (you can look it up in the lists if you like ;) ).
The international default is that highway=footway is restricted to pedestrians. This is also the case for 99% of country-specific values. A few places allow bikes under some circumstances and Brazil appears to allow mopeds. That's it.
Consequently **it is clearly documented, and universally accepted, that there is no need to tag access=private on a footway to prevent motor vehicle access**.
----
So these tags are unnecessary. A second question is whether they are *wrong*. In a technical sense, no, they aren't, for the reasons described by AndyS.
@AndyS.
In a community sense, however, intentionally breaking the visualisation used by the most popular renderers is antisocial at best and vandalism at worst. I would therefore expect that these tags should be removed and that such action, should it prove necessary (which I hope it won't), would be backed up by DWG.
As a postscript, @Sailor Steve, I think you would probably get a bit more sympathy if you could make your comments a little shorter and save us the moralising about "OSM will collapse under its own contradictions" and "loss of market confidence", which frankly just makes you look pompous.