The general rule is that if a homicide takes place during the commission of a felony, the participants in that felony are responsible for the homicide as felony murder, usually a lesser form of murder, with lower prison terms. Often, the question becomes whether they were still in the process of committing the felony. In the situation you postulate, the question is whether the felony had started already. There are 50 states each with their own history of determining the answers to these questions, so the case law is all over the place. You'd have to look at the state law first.
To answer your question of how it can be labeled a felony before the theft, you have to know what felony the prosecutors charged under, then look at any case law that says when that felony has occurred. If the felony was "entering a store with intent to rob", or "conspiracy" or something like that, it may have begun the minute they entered the store, or before they entered.
So if Brown and Johnson robbed a store and then fled the police thinking that the police were there to arrest them (even if the police didn't yet know about the robbery and stopped them for other suspicious behavior), wouldn't the fleeing from the police still be a part of the robbery?
-PJ