I'm guessing fivejedjon is going to say that I'm "on a hell bound train" in terms of lingustics but I'll ask this question. Here's what I heard in a series "Against the wall". Eevents in it take place in Chicago.
"So Rob came over. So what.? I'm single. It was a damn homicide convention for God sake! Ok, Yeah! We had sex. Since when did the IA( presumably Internal Affairs??) have the right to get involved in my sex life?" Hope there will be no hard feelings taken. Why would the speaker use the present simle in "Since when did ........."?
Semantics aside,as for "why" someone would use this phraseology in AmE we use "since when does/did/is/are" when confronted with something unexpected or unbelieveable or surprising.
Examples:
A friend arrives at your house wearing a Western-style scarf tied around his neck as part of his normal clothing. You personally think he's hopelessly out of style, so you remark: "Since when did bandanas come back into fashion? I must have missed that issue of GQ."
One of your employees arrives at your rather conservative office wearing flip-flops on her feet. As a way of telling her that such footwear is inappropriate you ask her: "Since when did shower shoes become part of the dress code?"
The present tense is possible too. Let's say a friend of yours, who has always travelled everywhere by public transport, suddenly rolls up outside your front door driving a Porsche. You might say "Since when do you have a car?" or more likely "Since when can you afford a Porsche?"
In Br Eng I feel that we would prefer a present perfect here': 'Since when have you had a car?'/'Since when have you been able to afford a Porche?'.
This would make sense (that is, one could make sense of it), but to my ear the present perfect woud sound better.
b
I would say the difference is in the time frame of the actual statement. If the statement were being made now, then "Since when does..." would be appropriate. However, in the book you quoted, and in most novels, the action takes place in the past so "Since when did..." would be appropriate.