The context of WWII is still with us today. I mean, it's still a relevant context. We aren't talking about the big band era here. We're talking about a war that still effects policy and feelings. Why do you think swastikas are outlawed in Germany? Because the context of WWII was so powerful that that symbol is still meaningful. So is the word "Jap" to a lot of people that lived through that. It doesn't get as much play in the West, that's for sure, but in the East I'm sure it's still a big no-no.
Irrelevant argument, the French Revolution, the American War of Independence or the Napoleonic wars also still have effects on policy to this day. And even bigger then WWII (some European countries still use the Code Napoleon in their city-policy and such, the American constitution has been unaltered to this day). But I don't think the negative slurs then are still being seen as insults now.
Sure don't use the word Jap to a actual japanese you just met, but really, using it in casual talking with friends, it's completely harmless. And there's nothing wrong with being courteous about mentioning "we don't use the word as an insult anymore, it's just a handy abbreviation". Hell, using the word in neutral or even positive connotations is a sign of social acceptance.
Also comparing it to putting other ethnics in the back of the bus is just wrong. That was a forced undemocratic practice, using Jap actually has a neutral function of abbreviation. It's more like file-sharing programs, yeah sure it can do harm (by supporting piracy) if used in that intent but it also has a major neutral and positive function if used as it was meant to be used (sharing free- and shareware).
So in a democratic rational sense, the use of the word jap is just an handy abbrevation as long as that is the intended use. Unless you're a WWII-veteran, that's probably the only use for it. If you get mentioned by a Japanese person that the word is offensive to them, mentioning the neutral function and use should be enough to keep using it as such. Trying to prohibit the neutral use is actually violating the right of free speech in a way.
Now offcourse, you do have a social cultural aspect on the other side and should that be ignored? It is offcourse the emotional irrational laden side and I always believe that if the intent is well explained (the neutral function of abbreviation) that should be that. If you get mad over me then using it in that function, the problem is mostly in your own camp. All that social cultural crap is a layer of varnish anyway that gets reapplied by every passing generation. What type of varnish depends on what functions we need fullfilled by our social cultural behavior.