Material witness warrants are why tourists should really think twice about the US.
"if it is shown that it may become impracticable to secure the presence of the person by subpoena, a judicial officer may order the arrest of the person" https://www.law.cornell.edu/uscode/text/18/3144
That means any witness who intends on leaving the jurisdiction anytime soon may be arrested. And once arrested, bail is difficult given that you are being held because you might flee. So any tourist who is victimized in the US, and may therefore be wanted as a witness, should seriously consider revisiting their travel plans.