Seriously NOT wanting to be argumentative, but bike is 5 years old, had an initial 3yrs Warranty, possibly 2, can't recall when they changed.
Three throttle bodies covered under extended warranty would imply that each lasted , on average, less than a year.
Surely as a replacement part they would carry an individual warranty for that replacement part??
Regardless of that the fact, your bike has had four TBs which seems weird considering many bikes survive on the originals.
Conclusion can only be one of the following:-
Rider doing something wrong, either riding or storing
Dealer doing something wrong at service time to cause problem
Dealer saying that something is wrong - because it will be covered by warranty

Rider is really unlucky and not subject to the law of averages - stick some butter in fridge.