Because they are Border Patrol agents who normally work out in the desert or forests. These are their regular uniforms.
ICE does interior enforcement too. I suspect these are not their regular uniforms. I am surprised they are not U. S. Marshals. That would seem to be the unit to defend these federal facilities. Did they duck out on this one?
Got it - but wouldn't clothing with, say, bricks imprinted on them be a better choice in an urban setting?