Fuel tank (And vents)

My wife bought a 2013 Ford Edge.  She bought it in
2016 and they, Ford, give you a 60,000 or 3 year
warranty.  It had around 21,000 miles on it when
bought.
The check engine light came on (02/20/2018) and
they told my wife it was the thing that vents the
fuel tank that set off the warning light (There was a
special word they used).  They say it isn't covered
by the warranty.  It seems like a straight forward
job and why does Ford, the parent company sell a
car that does this and then doesn't honor the
warranty or at least makes a warranty that only
covers certain things, few things. It should be 100
percent or nothing.
Thanks for reading
James