For one day tickets, yes, they sell a non-hopper ticket. All tickets of 2 days or more are automatically hoppers.
The dirty secret is that when DLR tried to charge extra for park hopping, nobody would pay it and DCA was empty. This was so embarassing to them that they started making all two day tickets have "free" park hopping to encourage people to go over to DCA. Over time DCA has developed more loyalty and many people (like me) feel it is not nearly as bad as many have claimed. But in truth it does have weaknesses.
IMO DCA is on a par with AK or Disney Studios - one day parks which complement the more full fledged parks.