You're right in theory.
But most of these errors are just details (eg method="GET" instead of method="get").
The W3 validator is very strict, so a lot of details will become errors.
Another example: sometimes a developer makes errors by intension, just because it will look the same in all browsers (closing some tags will generate an extra white space in older versions of IE for example).
So it has no use coding everything correctly while browsers don't parse them correctly.
There are more important problems that need to be solved by Canonical.
IMHO it would be a waste of manpower, which could be used to improve things for us, end users.
I don't see why we shouldn't. Its easy to find web developers. Not sure why Canonical couldn't hire one short term to fix this (may only take a few hours)
I would be surprised if this wasn't policy. The websites should be fixed to be standards compliant. Most fixes are probably simple. If there are parts of the Ubuntu websites that are not easily fixed, it should be a long-term goal to make them standards compliant.