Sounds more like based on alleged crimes previously committed by the pilot.
Without knowing more than I've read in the court papers filed by both sides in the SCOTUS case, this is what it sounds like (i.e., I'm speculating a bit) to me:
Based on information received from sources the state police considered reliable, they prepared an application for a search warrant, including the required affidavits laying out for the judge enough information to convince the judge there is sufficient probable cause of criminal activity to issue a warrant. The state police then wait until they became aware of another flight where they might catch the pilot in the act. (As an aside, the already-pending investigation led to indictments for felony importation. In that case, the customer pleaded guilty of felony importation; Jouppi plea bargained to a lesser charge.)
I think most here realize that I can argue either side of this one. Since this is a hot online topic, I'm trying to provide information and be as neutral as I can. Anyone is welcome to read the
papers filed in the SCOTUS case. I can easily see at least four different results, including those where either side wins on principle but loses in practical terms. With that in mind...
The present case is limited to the forfeiture. Forfeitures are subject to the 8th Amendment protection against unreasonable fines. This case is about what must be looked at in terms of what is excessive. Alaska (and other states) use what has been described as a "abstract" test. They look at the harm the legislation was designed to address. In this case a severe alcoholism problem in small, remote communities ("a community off the road system and home to fewer than 100 people" according to one of the filings). The states on the other side look at that, but, in addition, also look at the offender's specific conduct - this was misdemeanor level and the pilot can only be charged with one 6 pack out of the 9 gallons of beer on board.
The customer pleaded guilty to the criminal charges The pilot (and his company) were found guilty at trial. The pilot appealed the conviction and lost in the Alaska Court of Appeals. There has been no further appeal. The question of Jouppi's guilt is over.
In terms of the forfeiture itself, the history is interesting. Apparently, the judge did not want to impose the forfeiture. The judge originally read the forfeiture law as not applying because the airplane never got to the dry destination. In the first appeal where Jouppi's conviction was upheld, the Alaska Court of Appeals reversed the judge on the forfeiture, holding that, under the statute, delivery is not required. The Court of Appeals sent the case back to the judge to determine whether forfeiture of the airplane is excessive. Again, the judge refused to order forfeiture, saying it was excessive. In the second appeal, the Court of Appeals again reversed the judge, and the Alaska Supreme Court ultimately said it was not excessive. It's that decision which has now been accepted by SCOTUS.