Because the crime-related gameplay is an inconsistent mess and has been for years. You're allowed to deliver Combat Stabilisers, you're just not allowed to transport them.
Usually this confusion works in your favour: you're not allowed to transport personal firearms in many jurisdictions, but you are permitted to carry and use them. Even fewer jurisdictions permit the transport of battlefield heavy weapons, but they allow and encourage you to carry and use them on the outside of your ship.
(On the bright side, a Brewer Corp fine is about the least consequential sort, since the construction sites don't care and they don't own any normal assets)