USEF holds the trainer responsible for drug/medication violations. Not the rider, not the owner.
The owner usually has to pay a small fee to the show where the drug/medication violation took place but they are not suspended. The rider is not fined or suspended nor is the horse.
In a case where the owner and the trainer are the same person then the horses owned by the trainer are suspended because of the owner’s role as trainer.
As Janet mentioned, the reason many horses end up on the suspension list is because the owner failed to settle with a show or shows. That suspension is open ended till the debt is settled.
It is not unusual to see a suspended horse with a fairly current competition record.
You can often determine the reason for the suspension by reviewing the online USEF ‘For the Record’ reports of Hearing Committee decisions. But keep in mind that the period of suspension is not usually immediately after a ruling is made. It’s often a year after the date of the violation. For example, if a person is suspended 30 days for a violation that took place in August 2011 their 30 day suspension will start the 1st day of August 2012.