Give "premier league why have 2 goalkeepers on the bench" a Google and you'll see plenty of managers much more intelligent than me give compelling reasons for doing it.So it's a risk averse strategy to avoid damaging a players development, rather than a purely goalkeeping decision.
Do most premier league academies follow this policy ?
But at the end of the day, it's not an academy policy as you say, it's just the decision of the first team manager. What that boils down to is that for an academy player to get on the bench, he has to be better than an existing squad member. Convincing a premier league manager to leave out a professional for an academy player won't be easy given the average premier league manager is only 4 or 5 bad games from the sack.