It's kinda misleading. The admins tell you that you have to stay inside the bank under the whole robbery, while the rule clearly states that you must be in there when it starts, and nothing else.
Also, this discourages organized, thus effective, robberies, as 5 people stacked up inside the bank isn't really going to end well.
The problem with people covering the robber from outside the bank, was only when the police force were outnumbered, which happens rarely.