That's the justification they're counting on to not raise any suspicion in the general population. It's why no one is asking questions about such an unprecedented military occupation.
I'm curious to hear what reason you believe they have to deputize thousands of NG in DC to be US Marshalls.
My opinion? 2 reasons
1. The riots "touched them on their studio". The politicians and those who can call in the troops were spooked, so they are doing a knee jerk reaction for their own protection.
2. The world is watching DC more than anywhere else today, so may as well make sure it's a nice ceremony where no-one is hanged (hung? Can a grammar nazi please confirm the correct wording for hanging someone in past tense)