"And what they leave shall be eaten by the animals of the field" — the Torah establishes that shemitah produce left uneaten by humans may be consumed by wild animals. But the Mekhilta uses this verse to address a completely different question: is shemitah produce subject to the tithe?
(Deuteronomy 14:22) commands: "Tithe shall you tithe." One might think this applies even to produce that grows during the shemitah year. But (Leviticus 25:6) equates human beings and animals in the context of shemitah: "for you and your man-servant and your maid-servant and for your beast."
The Mekhilta draws the comparison: just as a beast eats whatever it finds on the shemitah year without tithing, so may a human being. The Torah likens man to beast for purposes of the shemitah. If animals eat untithed produce during the seventh year, humans do too.
This ruling exempts all shemitah produce from the tithing obligation. The shemitah year creates a special category: food that belongs to no one, subject to no owner's obligations, available equally to people and animals. The normal structures of agricultural law — tithes, ownership, distribution rights — are suspended. During the seventh year, the land returns to a state of primordial freedom, and its produce carries none of the legal burdens that attach to cultivated crops in ordinary years.