But they were the ones who controlled the Crusade. When not enough people showed up and they realized they couldn't get the money they were supposed to get for transporting the Crusaders, they demanded the Crusaders help them in their own, strictly Venetian, affairs - they made them put down a revolt at Zara, but that wasn't enough to repay the huge Venetian debt, so they made the Crusaders help Alexius take the throne in order to get paid.

Of course, then Alexius couldn't pay them, and had made all sorts of ridiculous promises (like bringing Byzantium into the Catholic Church's power) and when he got himself killed by his own people (something that shouldn't have been a surprise), the Venetians were central in urging the Crusaders to take the city so they could...get paid.

At every step of the process, it was the Venetians who directed the actions of the Crusaders, and at every step of the way, they were only concerned with enriching themselves. Sure, it might be better that the bronze horses from the Hippodrome were carted off to Venice, rather than smashed by rampaging Turks, but the Venetians helped ensure the rampaging Turks made it to Constantinople.

They don't deserve all the blame, but a lot of it? Yes.

Norwich's assessment, though, sounds a lot more like yours (i.e., he isn't as hard on them as I am).