The most important hints to the question are as follows, with #2 being the most specific:
1) patient reports pain with overhead motion and reports recurrent overhead motion during work. Overhead motion can damage the supraspinatus muscle due to impingement by the acromion.
2) Pain is worst with internal rotation of the shoulder - this is consistent with the findings of the empty-can test, which indicates a supraspinatus injury.
What is there that rules out deltoid? overhead abduction is >15' so shouldn't that point more towards deltoid?
"During strength testing the patient has pain and weakness with abduction, particularly with simultaneous shoulder internal rotation"
This is a descriptive way of describing Neer's impingement sign, which is a (fairly) specific indicator of shoulder cuff tendon impingement; the most commonly impinged tendon of the rotator cuff being supraspinatus of course.