I'm a former prosecutor and practice criminal federal law. I read the indictment, so let me sprinkle a few facts for you guys so you can remove your foil hats.
The prosecutor is not an elected Manhattan DA. It is an Acting US Atty (Audrey Strauss) appointed by Trump. She herself presented the case to the grand jury, which is rarely done. The SDNY is the premier US Atty's Office in the nation and can initiate a prosecution against just about anyone. But an indictment like this is definitely not done without knowledge and approval of the upper level attorneys at the DOJ in DC (a/k/a "Main Justice"). The investigation is over a year old, so William Barr has known about this investigation for awhile now and I guarantee Barr himself gave the final ok on it. It's not like he or the president first heard about this on CNN.
As far as the FBI or USPIS doing the investigation, it is not uncommon at all for the USPIS to handle a case like this. They handle a lot of financial fraud cases, especially internet based schemes like this.
From what I saw in the indictment, Bannon just about handed them a conviction with his emails, text messages, phony invoices, bank statements, credit card statements, etc. And right about now, the three schlubs whom he was involved with are all banging on the US Atty's office looking to give the DOJ the last 1% of evidence that they don't have.
Not sure I see anyone from the "thieving Left" in this equation.