What was it Ya'akov's business if the shepherds were idling when they shouldn't have been?
Rashi: Because if they were hired shepherds, they were wasting their employer's time (which is akin to stealing); whereas if they were self-employed, it was not time to take the sheep away from the pasture, in which they were depriving the sheep both of their food and by not watering them there and then. 1
And Tza'ar Ba'alei Chayim is Min ha'Torah (as the Gemara explains in Bava Metzi'a, 32b) - Or ha'Chayim.
And because he was sincere in his rebuke, the shepherds accepted it and did not accuse him of not minding his own business.
רש"י: ואם הבהמות שלכם וכו': למה הוסיף רש"י "ואם הבהמות שלכם"?
גור אריה: קשה שדי לומר "הן עוד היום גדול"? אלא התוספת של "לא עת האסף המקנה" באה ללמד ש-אם הבהמות שלכם וכו'.