And I guarantee you your programmers aren't working on your enterprise software outside of work. Conversely, your sales people are practicing the exact same activities (social interaction, manipulation, etc) outside of work on their personal time.
A prostitute would gladly sleep with a star NBA player for free, so should she/he not be compensated for sleeping with an aging/overweight and smelly client?
I enjoy playing high quality video games in my free time like Zelda. Should I not receive composition for bug testing the latest mind numbing Bejeweled mobile game crap at my job?
What I am saying is this... everyone interacts with other human beings. There is a large range in the quality of those interactions. Businesses must incentive heavily someone to make them actively seek out the most difficult and profitable of those human interactions.
Because someone chooses to interact with another human in their free time does not mean they shouldn't be paid for it.
Edit: As another post says, it's because sales people are only one step removed from the revenue-gaining process - they are often directly responsible for closing a sale, so they can easily claim credit.