..he would ask him: "Father, how are salt and hay tithed?" [Though, he actually knew that there is no requirement that these items be tithed.] His father would thereby think that he meticulously observed the mitzvos."
Many readers of Rashi use the story of the salt and straw to suggest that Esav was a pious fraud. Like the men who wear large shtreimals, but cheat on their taxes and skip davening, Esav, in their conception, was a master of deception, who used sincere-sounding questions to deceive his father. Indeed, Rash picks us on this idea by comparing Esav to a pig, a non-kosher animal that extends its forelegs as if to show off how kosher it is, fooling all who allow themselves to be deceived.
Midrash is not a monolith, and the midrash's view of Esav, especially, is complicated, and full of competing and mutually exclusive ideas. Though it is true that some of our Rabbis did think of Esav as a pious fraud, I will argue that the author of the salt and straw story decidedly did not.
First, let us recognize that midrash isn't history. It isn't telling us that such a conversation actually took place. If you went back in time, you would not find Esav and Yizchack discussing tithes. Rather, this story was created by the author of the midrash for the purpose of conveying an idea.
To understand the author's purpose we must begin with the blessings Yaakov received when he was dressed as Esav. They relate entirely and exclusively to the physical world: tal hashamayim ushmanei haaretz; the dew from the sky and the fat of the earth. Later, at the end of the story, when Yaakov leaves for Aram and is no longer disguised, his father blesses him again, this time saying: ve’yiten lecha es birkas Avraham; he passes Avraham’s spiritual legacy on to Yaakov.
It would seem from this that Yitzchak's intention was to let Yaakov stay in his tent and live a life of contemplation and study; while, Esav, the man of the field, contended with the world and provided for his religious brother. If, as most of our Rabbis say, Yitzchak was planning for the future of the Jewish nation when he blessed his sons, perhaps he was also setting up a sort of Holy Roman Empire (forgive the ahistorical reference) with Yaakov, as Pope, in charge of the nation's spiritual life, and Esav, as Emperor, responsible for feeding and protecting and otherwise sustaining the religious center.
Esav, having grown up in his father's tent, and eaten at his father's table, was aware of this plan, we can assume. The salt and straw midrash tells us that its author thought Esav was unhappy with this plan.
In the ancient world, salt and straw were important preservatives. Salt protected meat from spoiling, and straw was used as a packing material, or as insulation. Yitzchak planned for Esav to be Yaakov's protector, to serve as his straw and salt. The Midrash says Esav asked "How are straw and salt tithed (or "fixed" in the language of the original midrash) Conceptually, this is like asking "How are straw and salt brought into the relam of holiness?" By putting such a question in Esav's mouth, the author of the midrash is letting us know that, in his view, Esav wants something more. He doesn't want to spend his life merely sustaining Yaakov; instead, he wants a holy purpose, too.
I'll leave it to others to explain why, in the fullness of time, this more-positive image of Esav was lost. My guess it has to do with the fact that Esav was, at the turn of the millenium, linked with Rome. Before that association was made, I suppose, more positive opinions of Esav could be entertained. Not so once the Rabbis had paid themselves the compliment of associating the super-power of the day with their own great ancestor's twin brother. From then on Esav was evil, unmitigated, and unredeemable. The salt and straw midrash gives us a glimmer of another point of view.