Shambles, the tradition of having someone collect for the performer is called 'bottling.'
Much like a barker or shill at a carnival or sideshow.
It is an old and respected tradition that seems to be falling out of favor because of the experience you had in the subway.

No problem with someone collecting for the performer. It was just the nature of this operation. Perhaps some more details will help explain exactly my concerns?

The two enter the compartment and stay there until the next stop. These stops present the only opportunity for the buskers, or indeed anyone else from leaving until the train stops, as there are connecting doors but these are strictly only to be used in an emergency.
The time between stops will vary between 3 minutes or could be as long as 30 minutes. 5 - 10 minutes would be the norm however. For this period the bottler or bagman will make a point in this time of approaching everyone sitting in the compartment.

The passenger knows that should they chose not to contribute, when the bottler stands over them and presents the bag to them, that they will have to share to compartment with these two, until the next stop. Now if there are a few other passengers present, it is probably not too threating a situation but if there are few or you are on your own, perhaps it could with some justification be thought more than a little intimidating? That is why I say that the money collected here is probably not generally due to how much the passengers have enjoyed the music and not good busking practice.

I am not by any means a regular traveller on the 'tube' these days, so I have no idea how common a practice this is. My experience was a couple of years ago and took place in the middle of the day when the train was stopping outside in some of the less frequented stations. The same areas, late at night would be even more potentially intimidating.