Nice question, and explanation.

But: although the correct answer genuinely IS teh correct answer, the BoL reference given doesn't support it: that reference says "There is no single best approach for choosing an account type." and talks about the trade-off of having to register the service with the user account if you have network security, effectively suggesting that there are reasons why using the network account might be a better option (perhaps it would be if there were no other services on this server running under the network account, so that's not as silly as it sounds) - so in my view it would have been better to refer to http://msdn.microsoft.com/en-us/library/ms189964.aspx which is about the specific topic of SSRS service accounts and is much clearer in its reccomendations (in the "Choosing an account" section) and provides important information about circumstances in which there is no other useful option than a domain user account (Sharepoint integrated mode, constrained delegation).