I think the fact that United, like Liverpool, have lots of non-match going fans in every town and city across the country has a lot to do with it. As a local City fan, it's nothing to do with that and it's purely the local rivalry thing that shapes it for me - if you were bottom of the 4th tier, I'd still not want you to win so much as a throw-in (unless a United win benefitted City in some way). That said, I've got mates who have followed United all over the place and while we've been involved in some heated debates down the years, deep down there's a begrudging respect between us because we've all put a hell of a lot of time, money, and effort into following our clubs. One of them is a former United hooligan but is as sound as they come, and he's always looked out for his City-supporting mates.
But back to the glory-hunter thing - you're right that even match-going United fans get tagged with it and that must be annoying as hell (I know if someone accused me of being a glory-hunting City fan, they'd be swallowing their words within seconds), especially if it comes from someone who doesn't exactly attend too many matches themselves. A guy at work here is a United fan and has held a season ticket for many years. A few years back he was on a beano with some mates in York and they were watching a United game in a pub. An Arsenal fan who was in there piped up to him: "United fan eh? So how many times have you been to Old Trafford then?" to which he replied quick as a flash "More times than Ryan Giggs". Needless to say, the Arsenal fan soon shut up.
I'll also add that glory-hunting fans aren't confined to non-local areas. There are plenty of local glory-hunters - and City are starting to attract them in considerable numbers too - who have never set foot inside the stadium of the club they support and have no intention of doing so. As such, those non-local United fans that actually do attend matches deserve a lot more credit than the local non-attending glory-hunters for putting the time, money, and effort into following the club. I remember a post on here many years ago where a United fan from Plymouth was staying in Manchester after a home game. He got talking to a local United fan in one of the clubs in town who had never been to Old Trafford and was lecturing the Plymouth-based red on how expensive it was to buy a ticket.....while spending a fortune on alcohol.