For me the reason is simple.
If you talk about things that happened in the past, you use the past tense. This is how I learned the english language.
Even if I talk about a person who is here still, when you talk about things he has done in tha past, you use past tense.
You can't say HIM is holding his Leage of Nations Speech NOW, or tomorrow. No you have to speak in the past if you want to say it correctly. But of course HIM mind never changes. Today he would say the same things like in the past.
That is why we say HIM says that, the Bible says that, Christ says that. But just if you want to say it historically correct, you say HIM said that in this and that speech on this and that date.
I wouldn't say that speaking of HIM in past tense is disrespectful and means you say he's dead and gone.