Clearly the judge didn't consider it "completely legal", presumably based on 18 U.S.C. § 1512[0] which says the following may result in a fine or imprisonment (edited for brevity):

(b) Whoever knowingly uses intimidation, threatens, or corruptly persuades another person, or attempts to do so, or engages in misleading conduct toward another person, with intent to—

(1) influence, delay, or prevent the testimony of any person in an official proceeding;

(2) cause or induce any person to—

(A) withhold testimony, or withhold a record, document, or other object, from an official proceeding;

...

(d) Whoever intentionally harasses another person and thereby hinders, delays, prevents, or dissuades any person from—

(1) attending or testifying in an official proceeding;

[0] https://www.law.cornell.edu/uscode/text/18/1512