Some people do compete for speed or score in single player games. Arcade games have always had scoreboards, modern "arcade-style" games have online ones, and a community can turn any solitary activity into a competitive one:
http://speedrunslive.com/
http://speeddemosarchive.com/
Speedrunners are an exceptional case, but I think everyone gets a little annoyed when they look at a leaderboard and all the top players have scores of UINT_MAX or times of 0 seconds.
Obviously cheaters will find a way regardless of whether you give them the source code or not, I'm just saying dfc's concern is not totally ridiculous.