You are describing the No True Scotsman fallacy, which you might understand as a special form of equivocation (as in the good suggestion in another answer). NTS involves insulating a claim from counterexamples by appearing to use one of its terms (e.g. Capitalism) in a descriptive way, as describing and being made true by actual cases, but actually using it in a very restrictive prescriptive way. In this prescriptive sense, a writer allows that the term applies only when it makes the claim it is part of true.
For example, for the term Scotsman, the categorical claim might be "No Scotsman eats porridge with sugar." Consider the objection, "but my friend Angus MacDonald from Edinburgh eats porridge with sugar!"
For a reasonable person, that should count as a counterexample to the categorical claim. The writer should modify it or give it up entirely.
The writer commits the New True Scotsman fallacy when instead of giving up or modifying the claim in the face of the counterexample, he or she sticks to it by rejecting the counterexample – here by rejecting that Angus MacDonald from Edinburgh counts as a true Scotsman, as he can't be one (the reasoning goes), precisely because he eats porridge with sugar!
That is, the writer is actually using the term in such a way that it automatically excludes all counterexamples. It is being treated as true by definition. What's fallacious or misleading is that the claim is presented as a descriptive claim, when really it's offering a prescription for how to define its central term.