One man's hell is another man's heaven.
The fact that it's a punishment assignment for Scotty doesn't mean it is so for anyone else.
Besides, Keenser might have already been assigned to work with Scotty on his previous posting. After Scotty was posted to Delta Vega, Keenser might have volunteered to accompany him out of respect for his colleague. It's clear that Keenser likes Scotty and was sad to see him go...
(which brings to mind, what is Keenser still doing at Delta Vega? They don't expect him to work there alone, do they?)