Because they couldn't (or wouldn't) name the buyer. In order to have a transaction you need a buyer and a seller. And, politicians make these kinds of backroom trades all the time. If it's illegal why are the jails not packed to the rafters with pols? The whole thing was Obama getting rid of Illinois political rivals. The reason Jackson wants Blago pardoned is that his son also got taken out by Obama.
Jackson, Jr was an embarrassing screwup, using campaign funds to go on a shopping spree. He'd been given a Congressional seat that was a lock for the next 30 years, and had to give it up. He wasn't even bright enough to let others buy his bribes...
Obama had to throw Jr to the wolves, or start answering questions about a host of Chicago actions.
I grew up there. Lesson ONE in Chicago, is never trust an honest man. A fellow crook can be trusted to go blind when he sees someone elses's sin. And Jr knows that, as long as he took one for the team, he'd be covered.