You're right, nobody will be automagically freed as a result. That being said, pushing for amnesty, and pushing for pardons is completely how the law works, and very doable.
What is stopping a law from being passed that operates in a similar way to a pardon?
"All people convicted of X set of laws have their sentences nullified and this criminal conviction removed from their records" as a law passed, or something like that?
>What is stopping a law from being passed that operates in a similar way to a pardon?
Nothing. In fact[0]:
"As of this writing [12 March 2020], 26 states, D.C., and one territory have legalized or decriminalized marijuana to some degree. Eleven states and D.C. have done both. Seventeen states and D.C. have enacted expungement, sealing, or set-aside laws specifically for marijuana, or targeted more generally to decriminalized or legalized conduct. Four states have pardon programs for marijuana offenses."