The emails are all from October 2006 or earlier, back when the GOP was operating under the "Permanent Majority Illusion", without a care in the world of ever getting a subpoena.
They are still acting like they are in charge, but Congress is Democratic now, and the subpoenas are flying.

As far as executive privilege goes, Bill Clinton was forced to testify under oath, if you can compel the president to testify in a frivolous lawsuit, then the you can probably compel executive branch employees to testify concerning areas of legitimate concern before Congress.

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“ The subjects of every state ought to contribute towards the support of the government, as nearly as possible, in proportion to their respective abilities; that is, in proportion to the revenue which they respectively enjoy under the protection of the state.”

Adam Smith , The Wealth of Nations 1776

"We have always known that heedless self-interest was bad morals; we know now that it is bad economics"
FDR's second Inaugural Address