In our scrim rules, you can't nade a lone hog if there are others available. We've recently changed the scrim rules to make an exception on Infinity, that you can fight over a rocket hog even if there are chains available. I think it applies only to the start of the map. It hasn't been updated on the web page, but it's a scrim rule anyway, so doesn't apply to public servers.
In public servers, we would prefer if guests didn't nade lone hogs when there are empty ones available, but it's not an enforced rule - we let it go as long as they take the hog they kill for. And as long as they aren't camping, or walking right past other hogs, or making a habit to kill at the expense of racing. But it is acceptable for them to nade a lone hog if they intend to take it, especially if it's a rocket. Not just infinity, but any map.
If they are already in a chain hog, driving, they shouldn't stop and get out to attack you on foot. But if you both were jumping out of chains to try to grab the same rocket then I'd say it's ok. Depending on the timing of your particular situation, it may be this other guest went too far, or it may have been legitimate, I'm not sure.
