My guess is that airport and airline policy on apron trash is pretty clear - any amount greater than zero is too much.
But that's a guess. Probably a fair guess, but still a guess. There likely is a prioritization from high to low which starts on the runway areas, goes to the taxiways, and finishes up in the tow ways and gate areas based upon severity of a FOD incident in each of those places.
FODNews.com
In general, there's a committee of responsibility, which starts with the airport manager and includes the various tenants and airport users at commercial airports. Expert input comes from engine and aircraft OEMs and operators.
Just because ground crew are working a UAL flight doesn't mean that they are employed by that airline or that they're authorized to venture to adjacent gates on the ramp. It may not be a UAL issue at all, it may be a IAD issue, a training issue, a supervision issue, a union issue, any number of things. Could be any one of a number of policies in place for not venturing near an aircraft or gate one isn't actively servicing or picking up debris from a gate with a parked aircraft. Not to justify any of this, but frankly, I wouldn't know, and neither would you without making an inquiry directly. I've flown probably 4 flights in and out of IAD in as many years, so my sample size is a bit small.
One's view out of a scratched 15 x 30 cm bit of perspex is a snapshot, not a comprehensive view of airport operations. Genuine concern would probably be better addressed to the IAD airport manager in the form of a letter with a copy to the UAL operations supervisor with as much detail as you can manage.