Chulin 54b - Defining the word "רשאי'

The Node B'Yehuda has a famous teshuva where he explains why the halacha requiring one to greet their rebbi on the 3 regalim (pesach, shavuos and succos) isn't codified in Shulchan Aruch. The Nodeh B'Yehuda explains based on the gemara in kiddushin 33b that one is not allowed to stand up for their rebbi more than twice a day so that the honor being shown to a rebbi shouldn't exceed the honor being shown to Hashem through the acceptance of Him as king in the reading of Shema which is only twice a day. Similarly, when there was a beis hamikdash, there couldn't be a halacha requiring one to visit their rebbi more than 3 times a year (even though it is technically appropriate every shabbos and rosh chodesh - as the gemara cites לא חודש היום ולא שבת היום). Now that there is no beis hamikdash and there is no mitzvah to be oleh regel on yom tovim, the halacha of visiting a rebbi is completely inapplicable for the same reason so that the honor shown to a rebbi shouldn't exceed the honor shown to Hashem.

The Node B'yehuda certainly assumes that the definition of the term רשאי in the context of אין אדם רשאי לעמוד בפני רבו אלא שחרית וערבית שלא יהא כבודו מרובה משל שמים, has to mean - NOT PERMITTED, as it would be simply translated. However, Tosafos says that in hour gemara when it cites the phrase אין בעלי אומניות רשאין לעמוד מפני ת"ח בשעה שעסוקין במלאכתם, can even be speaking about when they are doing their own work so there is no issur to stand, but the term רשאי means obligated. Meaning, workers aren't obligated to lose money and stand for Talmidei chachamim while they are busy with their own work. Tosafos says that the term in the gemara kiddushin can be translated the same way, one is not obligated to stand for a rebbi more than twice a day, but would certainly be allowed to. But, in truth this may work even better for the Nodeh B'yehuda because there is no halacha that nowadays one is not allowed to visit their rebbi on regel, just that it can't be an obligation. It would work well since it is based on the source of one not being "obligated" to stand up for their rebbi more than twice a day - they are allowed just not obligated.

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On another note, the gemara concludes that we can't prove from our gemara that honor shown to those going to do a mitzvah should supersede the honor shown to talmidei chachamim, because it could be that the requirement to stand for those who bring bikurim is simply שלא תהא נמצא מכשילן לעתיד לבא - meaning that they must be shown extra honor in order to encourage them to invest the energy and time to come again. From here we see that there is an obligation on everyone not only to do mitzvos, and not only to abstain from preventing the doing of mitzvos, but to actively do things that encourage others to do mitzvos.