Why would the UEE allow such a ship to be sold to regular civilians? If interdiction devices are illegal, how is RSI selling them?

Just as UEE civilians have access to heavy offensive weaponry, they have access to QEDs for personal protection and for use in careers that support the Advocacy and navy. The hope is that we'll be able to create situations where the use of interdiction is sanctioned, such as working within a security force or defending a homestead from pirates.