Simple answer is that the Battle of Britain wa just that - a battle to defend the British Isles, mainly the south of England. As such, this fell to Fighter Command to provide the defence, in the form of fighter squadrons to combat the attacking bombers, and attempt to ward off the Luftwaffe fighters defending these bombers.
As Bomber Command were equipped with ...er... bombers, these were of no use in the defence. This Command was, however, engaged in attacking the Channel ports on the French and Belgian coast, where the invasion barges etc were gathering, as well as night (and some daylight) raids deeper into enemy occupied territory, and Germany itself. Although these operations took part during the course of the Battle, they are not, and can not be, considered part of the battle, being a separate campaign which was part of the overall strategic planning (such as it was) of the bombing offensive, which continued throught the war.
As Bomber Command did not take part in the direct actions against the Luftwaffe, over the Channerl and the British Isles, they can not be considered to have been part of the Battle. However, for the purposes of recording the history of the period, the Order of Battle of Bomber Command is normally included in books dealing with the Battle.
A poor comparison could be that the Royal Navy were involved in the Battle of Berlin - they were active at the same time, but not over Berlin !