MO. Conservation dept. frown's (citation) on removing plant's from one body of water lest they find their way into another body of water where they may not be wanted.
Or when they become invasive or too much for your tank/my tank, and then get tossed.
Or if sold or transported across state lines.
Rather than accept assurances that none of the afore mentioned will happen,they choose to err on the side of caution and forbid it.