Mate, that's not cheating, that's diabolical work on the part of the linesman
What do you suggest Carroll should have done... go up to the referee and admit it was a goal? If he does that, he's out of a job... Alex Ferguson would have subbed him immediately, and then fired his arse as he crossed the chalk!
Yes, I do, since the referee and his linesman hadn't spotted it, and, as a matter of fact, it is cheating. Roy Carroll cheated, which is why he quickly and sneakily glanced at the linesman. Seeing as he saved United two points, why is it the one game during his career at United that he'd like to forget? That's not a rhetorical question, either.
I also reckon if the 'great' Ferguson had reacted as you suggest, with Carroll being immediately subbed and sacked, and him now out of a job, as you put it, basically for being honest, that would have come back with a vengeance to haunt Ferguson, and been one hell of a burden for him to carry for the rest of his life. What, sack a man for being honest? I wouldn't want that on my C.V.; would you want it on yours?
I venture to say Ferguson would not have the 'Sir' in front of his name now, if he had reacted as you suggest.
Referees and linesmen missing something that it can be argued that they should not have missed, be it football or rugby, or any other sport, for that matter, does not make them responsible for what they have missed happening. Maradona's 'hand of god' 'goal', proves him to be a cheat, but let's not forget that the referee and linesman of that game, also missed it. Is their incompetence not greater than those at Old Trafford, since it led to a goal actually being given, yet I've never heard of their being criticised, or the referee and linesman who failed to see the disgracefully cowardly tackle on O'Driscoll by the two All Blacks? Each day those two 'men' wake, they should thank their lucky stars they didn't put him in a wheelchair for the rest of his life.
Although we can criticise the referee and his linesmen for their failings, that's a viewpoint that can be leveled at the officials of any sport on the planet, but unless you can prove corruption at that officiating level, the officials can't and shouldn't be held responsible for what the actual players and participants do.