"Allow[ing] the terrorists into the building" doesn't follow from a single shot of opening a gate. For all I know, law enforcement is funneling people away from a position that they recently decided should be cleared. I have no idea (and as far as I can tell, neither does anyone else here).

That shot is right in front of the building basically. The police opened barricades to let people even closer to the building. Unless the police has some sort of amazing 4D plan they are not letting us in on, which is not likely given the events of today, there is no strategic reason to clear that area. They're just letting the protestors in.

The last few seconds very clearly shows more people already "let in" on the other side of the gate. More context is needed.