As stated above, it does depend on the state, but let me pose the situation in another way. Suppose it was a private home, an the owner left the door unlocked at night when he went to bed. Can I walk into the house and only be considered trespassing? The homeowner's failure to lock the door is not legal justification for entry, as we have a right to privacy the courts have consistently upheld.
When we talk in a legal sense about private property, and especially in the case of a business where property is maintained and a target for thieves, "breaking" the plane of the doorway outside of business hours would most likely be considered breaking and entering as well, and/or burglary. If someone is inside, then most likely robbery would be the charge.