This clause is overly restrictive. The authors appear to be fixated on people writing public policy for the public web. There are other use cases which support cherry-picking requirements with full selectivity.
One of these use cases is where a web development organization is subcontracting for media fragments -- icons and background images or the like -- from a subcontractor and writes a standard for acceptable data packages which requires metadata to go with each including provenance, sample ALT text, etc. In this case there is no reason that the customer organization should have to require all of Level A on the piece parts purchased from the subcontractor -- the purchasor will take care of the other aspects before putting the assembled web content out on the web. This requirement as now stated would make that a violation of the Recommendation.
Soften the language from an imperative to a Recommendation.
Don't say "you mustn't cite arbitrary subsets," but rather say "if you cite only a subset of the Level 1 Success Criteria, don't represent this as WCAG 2.0 Conformance."
-- alternate wording...
W3C does not regard satisfying a profile of success criteria that does not contain all the Level One success criteria to merit the term "conformance to WCAG 2.0"
Unless a specification or policy requires at least conformance to all Level 1 Success Criteria, do not represent that policy or specification as implementing WCAG 2.0 conformance.
We have made conformance claims less regulatory and more descriptive, that is, a conformance claim describes what is conformant to the guidelines. We think it is more appropriate for policy makers to determine appropriate exceptions.
We have taken your suggestion and addressed the issue by making it explicit that Level A is the minimum level for which WCAG 2 conformance can be claimed:
For level A conformance (the minimum level of conformance), the Web page satisfies all the Level A success criteria, or the page satisfies conformance requirement 4.