Just addressing your point about the difference in perception, I think the main reason is you just compare your team's player at their respective position to other teams. That is how you get the standard to judge them by.
Just given the nature of football, quarterbacks typically will have a lot more turnovers than a RB. Interceptions are on the table whenever you pass the ball, but hopefully your QB is good enough that the risk/reward ratio is in your favor and you win much more than you lose on forward passes. I think we have that situation with Romo, especially with the understandable 1st game hiccup of 3 interceptions.
Halfbacks carry the ball for a living, they should be really good at it when they reach the NFL level. Most starting RBs might fumble once or twice in a season. Murray should fumble less, especially if he wants to get paid as an elite RB.
Turnovers are bad for your team no matter what, but given the standard of the league and just the nature of football, an interception by your QB is easier to swallow than a fumble by your RB.