Still wonder what prevents, say, a wide receiver from running out of bounds on a route, though. Heck, the tackle was made from out of bounds in this instance - Fitzgerald never got back into the field of play.

Yup. Same reason a wide receiver can't run out of bounds and come back into the field of play and make a catch.

It comes up most frequently on kicks because kicks are one of the few times a player can run 40+ yards in a line on a play (and because other players can block you and push you out of bounds).

From MMQB:On kickoffs and punts, players cannot run out of bounds unless they are blocked out of bounds. On all other plays, there is not a rule about where they may run.

(Starts at "PLAYERS CAN RUN OUT OF BOUNDS ON NON-KICKOFFS").

Harrison should have been ejected. It was a pussy move and was clearly shown on the replay.

Sorry, I don't buy it. In fact, the blocks weren't even illegal.

The refs penalized Harrison because they think he continued blocking (pushing, shoving) after the play ended. That's debatable since you can see the players still running in the background).

The blocks themselves were perfectly legal.

Is there a rule about not running out of bounds on a interception return and you are trying to tackle the interceptor?

Yup. Same reason a wide receiver can't run out of bounds and come back into the field of play and make a catch.

It comes up most frequently on kicks because kicks are one of the few times a player can run 40+ yards in a line on a play (and because other players can block you and push you out of bounds).

But you are not biased or anything?

Who said I wasn't? I'm a Steelers fan. I don't hide that.

Only thought it was on kicking/punting plays.

Nonetheless, a very enjoyable Superbowl, except for all the penalties.