Hilchos Shemittah v’Yoveil 4:3

We have seen that the only shemittah produce that one may eat is the fruit of trees and herbs that aren’t planted by most people. However, vegetables that people commonly plant in their gardens, as well as grains and beans, are prohibited by Rabbinic enactment. Gathering such produce renders one liable to the penalty of stripes for acting rebelliously.

Hilchos Shemittah v’Yoveil 4:4

If safiach grows in an untilled field, a field that was plowed, a vineyard, or a field where crops had been planted (in the sixth year), it may be eaten. The Rabbinic enactment against safiach was not applied to these places because they’re not places where one would plant. An untilled field is “off people’s radar,” a plowed field is meant to remain fallow, no one would plant in a vineyard as that would render it prohibited, and safiach would ruin a field that was already planted. Similarly, one may use straw that grows anywhere during shemittah as no enactment was made to prohibit it.