"Why on earth would Congress now create barriers to investigation and subpoenas of a member's spending records?"
The political act of the originary word behind the USA: ** colonization** (in this case another version of "history" ... aka 'precedents'). The colonizing premise is always hunting for the least obtrusive colonizing by ideology - whether by elimination or authoritarian obfuscation.
Consider Trump. Muddy the waters, churn, churn, churn and then make the people think he and his ilk are not forcing a 'cry uncle' situation. As an oligarch's oligarch, Trump is a dream-come-true for the extracting classes to suck down to the last drop, during an epoch when parasites are defining the discourse.
Case in point: Sessions hearing and the question about immigration. The situation of immigration is, I would submit, due primarily to the parasitic activity of the US corporate interests under the United Fruit model (colonization, dehumanization, extraction, ethnocide, genocide, ecocide under monoculture for export). But during this epoch those connections will not be made. Why? Because pitting peoples against peoples is entirely too profitable to those who benefit from the campaign contributions and revolving doors. But a day is coming when the stunningly narcissistic stupidity of the model will fall down around the ears of those who have fomented it.