I agree that it is not the players responsibility to avoid us (with the exception of a TE trying to scrape off me to get free).

Ususally I hear from LBs complaining that I was in the way of their path for a tackle (of course the pile is usually already going down, but they believe that they would have made a difference).
My response is
'I'm not really here. I'm just air. Heavy air, but air. You go where you think you need to.'

I do my best to not be in the way, but have been wrapped up in tackles a few times. I appreciate it when the player appologizes afterwards, but otherwise I don't expect anything else out there.

The camera view looking into end zone gives a different look at this. The play basically went to the right, broke down and the QB scrambled to the left. The umpire pivoted to look into the line play to his left. When the QB took off, the umpire shuffled to his right to give the QB room to cut up field. When the QB cut east-west into him, he reacted and protected himself proactively.