Ouch.
I've had the pleasure of sitting in for a few pushbacks, and to be fair, the environment is totally different to that in the Flightdeck. it's a worthwhile experience, if some of us can belittle ourselves to experience it!
One possible thought as to why some Ground crew ask for brakes so promptly is they are aware of the consequences of delay penalties and that the signal on ACARS or whatever goes with brake release. Still totally bad SOP to push the flightcrew along though.
I have experienced it too, but didn't comply.
Like many aspects, the job of moving customers by air is a TEAM EXERCISE. All of us, tuggies, dispatchers, loaders, ATC and pilots and cabin crew all have a part to play, but none of us can do it alone. Pilots aren't necessarily the best paid of those mentioned either!