Barca screwed up and it wasn't in the second leg. They were mostly poor in the first leg, and the scoreline in the first leg allowed Inter to play the way they did in the second game. The way inter played is a valid tactic, in this one specific situation. If the first leg had been 0-0 and they played like that, they would have been beaten playing like that.
Even so Barca were truly terrible and should have still won last night. It took them the best part of what, 70 minutes before they even attempted a long shot, and it was Toure. Inter weren't being forced out of position or closing down from the half way line because they had no need to. Barca were ONLY looking for a pass through the defence, they weren't looking for the right pass till 10yards outside their box and they weren't providing a threat elsewhere. If they just started peppering the goal from range early on, they'd force a couple of saves and might have got lucky with the ball dropping for someone.
But most importantly as the danger increased, Inter would have been forced to close down players who even LOOKED like they might shoot, and that in itself opens up space as players rush to close down a shot.
Thats the problem, they didn't look threatening, Messi didn't shoot from 30 yards, so he no one closed him down. They were one dimensional and poor throughout. There was no urgency with their play and very few people actually tried to run at the back 4, because they were allowed, essentially to walk up about 10 yards out, they weren't ever attacking at pace. Neither did they try to draw Inter out at all to create some space in behind, they really tried nothing different all night.
Had it not been for the ill timed Cordoba substitution they wouldn't have scored at all.
As for a results driven business, you're mistaken, if Inter went on to become the CL winners, and won it every year for the next 20 years playing that exact same way, fans would not pay £400 for a ticket for the final, fans wouldn't even bother going to the final.
Its an ENTERTAINMENT driven business, because without any entertainment, no one would watch it.
Sure the odd team can get away with being overly defensive and dull as crap, but Chelsea hardly became the best supported team in the world while under Mourinho. Likewise all over people love to watch Barca and Real, while their Italian counterparts in the past decade simply don't get the same worldwide support.
Attacking and exciting football, means money and is the end goal for every board of directors and manager. Barca, Utd, Real, Arsenal, would have significantly less fans and FAR less income if they played like Inter did last night.