Unfortunately whilst the majority of GA pilots are very good there are still quite a few that don't understand the requirement for a clearance to enter controlled airspace. I've witnessed many cases where a pilot wrongly believed he could cross controlled airspace simply because they had checked in on the frequency.
This has led many units to use the phrase "remain outside controlled airspace" in an attempt to stop infringements, whilst personally I don't agree with this being used on every transmission you can understand why some controllers do in an attempt to cover themselves.