Changing trajectory means changing orbit and changing orbit takes energy. The presumed (and soon to be legally mandated) end of life goal of all smallsats and cubesats is to burn up in the atmosphere. You can do that by reserving a last gasp of propellant on the satellite to lower the satellite's orbit, but that assumes that there is a thruster on board somewhere. A lot of small satellites don't have thrusters. The drag sail approach is nice because it's passive, and all the energy required is collected from atmospheric impact rather than stored on the satellite as a propellant of some kind.