Vayeitzei, Yaakov, laborers, and birkat hamazon

(43) So the man grew exceedingly prosperous, and came to own large flocks, maidservants and menservants, camels and asses. (1) Now he heard the things that Laban’s sons were saying: “Jacob has taken all that was our father’s, and from that which was our father’s he has built up all this wealth.” (2) Jacob also saw that Laban’s manner toward him was not as it had been in the past. (3) Then the LORD said to Jacob, “Return to the land of your fathers where you were born, and I will be with you.” (4) Jacob had Rachel and Leah called to the field, where his flock was, (5) and said to them, “I see that your father’s manner toward me is not as it has been in the past. But the God of my father has been with me. (6) As you know, I have served your father with all my might; (7) but your father has cheated me, changing my wages time and again. God, however, would not let him do me harm.

(7) Just as the employer is warned against robbing the wage of the poor workman, and against delaying it, so is the poor workman warned against robbing the employer by idling away his time on the job, a little here and a little there, thus wasting the entire day deceitfully. He must be scrupulous throughout the time of work. Also, he is required to work to the best of his ability, as the upright Jacob said: "I have served your father with all my strength" (Genesis 31:6). For this reason, he was rewarded even in this world, as it is written: "The man became exceedingly rich" (30:43).

The laborer is warned against wasting a little here and a little there- rather he is required to be strict/precise with his time, because [the sages] already decreed regarding the fourth bracha of birkat hamazon, that he should not say it and so is required to work with all of his ability, like Jacob...

this is the language of the Tur- that the sages already decreed regarding a zimun and the fourth bracha etc. And regarding a zimun, we do not require a laborer to wait for others, so as not to nullify his labor (waste his time)

(1) The order of the Grace after meals is as follows: The first blessing is that beginning "Who feedeth"; the second is the blessing for the Promised Land; the third, that ending "Who rebuildeth Jerusalem"; the fourth, that beginning "Who is good and doeth good". The first blessing was instituted by Moses, our teacher; the second, by Joshua; the third, by David and Solomon, his son; the fourth, by the Sages of the Mishna.

(2) Laborers when taking a meal during the time that they are working for an employer, do not recite the blessing before the meal, so that they may not neglect his work, and only recite two blessings after it. The first of these two blessings is recited in its regular form. The second begins with the blessing for the Promised Land, includes a reference to the blessing, "Who will rebuild Jerusalem" and ends with the concluding formula of the second blessing, "Blessed art thou, O Lord, for the land and the food." If however the workmen's sole wage is their board, or if their employer takes the meal with them, they recite all the four blessings of the Grace after meals in their complete form as others do.

They don't say a blessing beforehand- because it's not from the Torah

But they bless afterwards- because it is from the Torah.

And they include [the third blessing with the second]- two blessings for the price of one, because birkat haaretz is similar to binyan yerushalayim

We change the second and third even though all three blessings are dioreita, which shows us the power of the chachamim to uproot something that comes from the Torah!