Because they're daft. Their buddies said 'well it's dilates blood vessels (it does) so it's gotta be good for the pump right?'.
Not only is there a disconnect (it's not that big of a deal) between a 'pump' and muscle growth it never occurs to those using something meant to help with erections for a bicep pump end up over using the drug.
What they gonna do when they need help getting it up? Use lolly sticks and a rubber bands??