A primary principle of Torah jurisprudence is that agreements are presumed to be according to local custom. This is learned from the mishna in Bava Metzia which states that when we hires workers, they may begin and end work at the hours customary in their area; likewise, if the local custom is to provide them with food then the employer is obligated to feed them (BM 83a, SA CM 331).
The mishna continues that if the employer offers to feed the workers in a place where the custom is already to give them a basic meal, he is obligated to give them a full and satisfying meal so much so that Rebbe Yochanan ben Matia opined that "even if you give them like Shlomo in his day, you have not fulfilled your obligation, for they are the children of Avraham, Yitzchak, and Yaakov." But most Rishonim rule in accordance with Rabban Shimon ben Gamliel who states that agreeing to provide meals doesn't add to the existing custom but merely restates it (Beit Yosef).
The gemara continues further with the theme of adhering to custom: "Rebbe Tanchum ben Chanilai said, One should never deviate from custom, for we see that Moshe ascended on high and didn't eat bread, whereas the angels [who visited Avraham] descended below and ate and drank" (BM 86b).
It is possible to view these two aggadot as presenting two different approaches to the idea of custom. The approach of Rebbe Yochanan ben Matia views custom as merely an implicit agreement, a widespread understanding. Really, any member of the Jewish people is worthy of a meal fit for a king; if we offer them "a meal" of our own volition, nothing less will suffice. But given that local custom is to provide a minimum amount, we may assume that the workers don't demand any more and so the employer is exempt.
By contrast, Rebbe Tanchum ben Chanilai views custom as something of inherent importance and suitability. Moshe could easily have explained to the angels on high that he is only a human being, and thus he is compelled to deviate from their custom by eating and drinking. But the heavenly dwelling of the angels is a place where eating and drinking are completely out of place; as the commentators point out, it is a realm of the intellect and the spirit, which are eternal and not dependent on material nourishment. The angels also could have found some excuse not to eat with Avraham, but they realized that in this world of matter the only way to honor someone is with the benefits appropriate to this world, including food and drink. Thus, Rebbe Tanchum ben Chanilai urges us to never deviate from custom.
The Tur writes that "It would seem that halakha is according to the first opinion, but the Rama [Rabbi Meir Halevi Abulafia, a prominent Rishon], ruled according to Rabban Shimon ben Gamliel." The Beit Yosef adds that most other Rishonim also seem to rule according to Rabban Shimon.
The Bach explains that there is a firm basis for ruling according to the first opinion. For one thing, the mishna provides a logical explanation: since they are already entitled to a meal due to custom, stipulating a meal would seem to imply an addition. Rabban Shimon ben Gamliel merely states, "All is according to custom", without an explanation. For another thing, the gemara discusses this opinion, and on the whole discussions are generally recorded on authoritative opinions.
Perhaps the aggadot themselves lend support to the consensus ruling. Rebbe Yochanan ben Matia brings a proof from the story of Avraham, who slaughtered three animals in order to feed his three guests. We see that in the house of the Patriarchs there was a custom of having splendid meals.
Yet Rebbe Tanchum points out that this very story teaches us the immense value of custom, for the entire meal would have been superfluous if not for the fact that Avraham's guests, the angels, were scrupulous to adhere to local custom and take part in this meal.
In a technical, legal dispute, aggadic stories are not appropriate proof texts. But in this case, the entire question is one of logic and reason which approach to custom is better defended? The Biblical stories of Avraham and the angels, or of Moshe ascending on high, are valid instances of the inherent importance of conforming to certain accepted standards of our surroundings.