I found a suitable answer tonight, so let's go to the 'source of the sin':
"Send forth men, if you please, and let them spy out the Land of Canaan that I give to the Children of Israel; one man each from his father's tribe shall you send, every one a leader among them," (Numbers, 13/2)
The implication of those three italicized words seems to be, "You can send spies but the decision to do so is yours." This wasn't a command, but a choice and hence Moshe has to bear some responsibility for the disastrous consequences of the mission. For more on this, I'll quote The Stone Edition Chumash:
As explained by the Sages and Rashi ... the people came to Moses and asked him to dispatch spies to reconnoiter Canaan and report to them. Moses consulted God, Who said, "I have told them the Land is good. [But since they question Me], I will let them test My veracity, at the risk of being misled and losing their chance to enter the Land." Although Moses apparently approved the demand, he actually hoped that his agreement would dissuade the people form pressing the request. The Sages offer a parable: Someone wants to buy a donkey, but says that he must first test it. The seller enthusiastically agrees. "May I take it to both mountains and valleys?" "Of course!" Seeing that the seller is so confident of his animal's prowess, the buyer decides he has nothing to fear and forgoes the test. He buys the donkey and is very satisfied. So, too, Moses thought that his willingness to let the people have their way would convince them that they had nothing to fear.
Interestingly enough, the same attitude (putting faith in the people to make the 'right choice') also cost Aaron his chance to enter the Promised Land. During the Golden Calf debacle, the commentators explain that Aaron took part in the incident only to play for time in hope that the people's enthusiasm would wane or that Moshe return from Mt. Sinai's summit. Despite this, the Torah still says Aaron "fashioned it [the Golden Calf]," (Numbers, 32/4).
Based on the examples expressed above, Abarbanel's commentary seems to hint that both these great individuals seriously erred as leaders. Instead of making decisions that may have been unpopular with the masses (or even result in their death, as per Aaron's worries after seeing Hor murdered - see Rashi's commentary to Numbers, 32/6), both showed bad judgment that caused catastrophic results for the Nation of Israel.