There's no such law. Mueller couldn't indict because he had nothing to start with. His leadman Weissman was doing all he could to cover for Peter Strzok & Liza Page by scrubbing their cellphones. Weissman threatened 500+ persons interviewed with perjury traps & years of litigations into bankruptcy so long as they fabricate stuff unfavorable to the administration. Despite millions of documents from 2+ years of unfettered investigations, interviewed 500+ persons of interest or witnesses, scouring through decades of taxes, secret surveillance and emails digging for any dirt from years before Trump's candidacy. There were no bleach bit of secret servers, hard drives & cellphones smashed with hammers.
Adam Schiff & Nadler's predictions about concrete evidence & indictments went dud. Mueller's investigation & report is all about assumptions created by nefarious operatives pushing the dossier narrative. There was no collusion or crime so Mueller couldn't conclude on the obstruction simply because they assume as they lack evidence or had none as to why they resorted on the fabricated dossier. In the end the Only Crime Mueller Team can come up with is perjury traps & Logan act for 6 people.
Lack of Evidence or proof of fabricated evidence by the prosecutor leads to acquittal & reprimanding the prosecutor & staff. Andrew Weissman, Mueller's lead attorney, was reprimanded in 1997 for withholding evidence. By a vote of 9-0 the Supreme Court overturned the Anderson conviction that Andrew sought to obtain through nefarious means.
Mueller indicting Russian's was a stunt knowing full well that they wont be questioned nor will they be talking to investigators. Mueller couldn't even definitively state that 13 Russians interfered, only Mueller & his team assumed they interfered & assumed that it was at the behest of Russian government. Can assumption hold up in court? Could they be rouge agents trying to cause chaos & disfunctions between the US & Russian relations, or are they just messing with people.
Mueller lied about WMDs to snugged the US into war & his prosecutorial team lead by Andrew Weissman scrubbed Peter Strzok's & Lisa Page phones to hide evidence. They are covering up corruption in the FBI by partisan or narcissistically corrupt zealots like James Comey on a high horse of hypocrisy collaborating with the DNC, Fusion GPS, & Christopher Steele .