I suppose you're right there. I do tend to position myself at the closest defensible spot to the enemy base so I can make returns if I hit an enemy capper before he gets there, but it's pretty difficult to be effective in that aspect. As for not being something a team can afford, that is true in smaller games with few people, but if it's a packed game then it's pretty inconsequential. There generally will be plenty of others serving that role, even to return the flag after I pick off a capper or assist in doing so.
In a packed game, you're still 1/8 of your team, and you could very well be the difference between winning and losing. A dedicated offense role usually has three main duties: 1. Make sure your team's capper can get away with the flag (distract/kill chasers), 2. Try to return your team's flag, and 3. Be there to make an emergency grab if needed.
As an O sniper, you can be very effective at #1, perhaps better than an LO in some respects if you can get folks to chase you. However, a laser rifle isn't going to knock a HoF off the flag stand at that crucial moment.
As for #2, you can assist others in doing the job, and you probably can be fairly helpful. But spawn times are fairly short, and you aren't there to control how the flag is moving on the ground. Returning often boils down to navigating the mad scramble for a loose flag.
#3 isn't usually possible for you at all.
So really, you're running at about 1/2 of the potential effectiveness of a regular LO. There are other factors at play, sure, but the inability to make any kind of first-hand action concerning flags is a pretty big inadequacy.
Of course, sniping is going to be a somewhat limited or support role no matter what position you play. But on defense, you'll be available if someone needs to pass the flag, and you'll be able to change quickly to a more suitable class if needed. I think it's undeniable that you can be a much bigger help playing as defense if you're going to snipe.
I'd hardly call it abuse. But I do see your point. I've been playing this a long while and have dealt with it about that long. And you are right, my main job is as an irritant and distraction, no matter how many cappers I pick off. I do find though that the average rager would still rage, no matter how I killed them. Osniping does tend to irk a few people in addition to that, but for the most part, ragers are ragers. I do d-snipe sometimes, and I get a lot of the same reactions. To some, one can't do anything right, and osniping is just something to latch onto. If it's not that it's sniping in general. If it's not that, it's rocket spamming. Or jumping around too much. Or hackcusations (I get a ton of those.)
Agreed: contempt for being sniped isn't exclusive to offense by any means. And people often get irritated by being OD or combo blocked in much the same way.
In the case of being sniped, OD blocked, or combo blocked, there is often very little that the victim could have done to avoid being hit or stopped, or if there is, it is often not as obvious as other situations are. I believe the only real satisfaction or fun that comes from a failure in game is being able to look back and realize the mistake you made and what you could have done different.
Sniping on defense can be used to irritate and distract, sure, but the main reason is to fight off incoming enemies. But with O sniping, people feel the same as above, but they are made to experience it for a less obvious or immediate reason, and that makes things all the more unsatisfying. So no, I wouldn't say that O sniping is just something people latch onto to justify contempt for sniping in general, though I'm sure that some do.
The point is, a certain aspect of maturity should still be expected. I do realize that osniping as a tactic,, while I hesitate to call it immature in its own right (at least not how I apply it) takes advantage of immaturity. I do think you'd agree that people should still be reasonable and play by the rules, something you've done well. There is nothing wrong with coming to kill me if you're on the other team, (apart from helping the diversion argument.

)
I could just as easily argue that an effective tactic is no reason to be unreasonable, or miserable for that matter. I am very used to being disliked, and that's fine, I just expect people to handle that in a mature fashion- I don't think anyone here would really call that unreasonable.
I agree. But this is the internet.