A Delta flight was recently forced to turn around an hour after take-off when maggots fell from the overhead compartment onto passengers sitting in the economy seats.
The flight on Tuesday 13 February was transporting travellers from Amsterdam to Detroit, Michigan, when a passenger’s suitcase containing rotting fish was in the overhead bin and opened up resulting in maggots falling onto passengers and the plane turning around.
After the plane landed all passengers were instructed to de-board the aircraft and the suitcase containing the fish was placed in a bag to be destroyed. The plane itself was also deeply cleaned.
Actually they are paid for that. One of the most important jobs of US Customs is agricultural protections and checks and I would assume the same is true for other countries. No one wants invasive species if they can help it.
Sometimes a country's customs screening is in a foreign country, but US customs does not have a preclearance office in Amsterdam. Countries typically don't have screenings for people leaving the country.
Today, CBP has more than 600 officers and agriculture specialists stationed at 15 Preclearance locations in 6 countries: Dublin and Shannon in Ireland; Aruba; Bermuda; Abu Dhabi in the United Arab Emirates; Nassau in the Bahamas; and Calgary, Toronto, Edmonton, Halifax, Montreal, Ottawa, Vancouver, Victoria, and Winnipeg in Canada.
Legend has it the UK (I think Manchester) was going to sign up to it but the US insisted their officers have guns, which the UK authorities didn't agree with.
When I fly to the US from Toronto I clear US customs in Pearson Airport, before getting on the plane. If I were to sign up for a Nexus pass I'd be able to pass through US customs even faster, but I don't travel often enough for it to be worthwhile. Not sure if either of those matches what you mean by pre-clearance.