(See also part 1, where I talk the signaling effects of grades for the two professions.)
Two more reasons why grades matter in law school but not for journalism school: (1) small variances in skill are more important for lawyers and (2) young talented lawyers are easier to retain.
A small skill difference in a trial lawyer can mean the difference between a legal victory and a defeat, often with millions of dollars at stake. Thus, it's worth trying to get metrics, such as grades, on the skills of prospective lawyers, even if they are imprecise.
On most traditional newspaper beats, a highly talented journalist isn't much more valuable than an average journalist. Having the best reporter in the world on the Manatee County, FL, courts beat won't generate much more revenue for the paper than an average reporter would. The potential audience for the subject is only so big; this was especially true before the Internet and widespread syndication. There's less of an incentive to hunt for the best reporters out of college, especially based on such an imprecise measure as grades.
The true cream of the reporting crop can be extremely valuable, but once these reporters have demonstrated their talent, they have every reason to leave for bigger newspapers. There, they can earn more money, reach a wider audience, and live in a bigger city. The small papers that hired them out of college don't have the resources to make an equally appealing offer. Even if small papers are successful in hiring the very best journalists out of college, the papers will have a hard time retaining them.
On the other hand, law firms can more easily hold on to young talent. A young promising lawyer may very well stay at the firm that hired him out of college, as he will begin to gain seniority and work his way up to partner. Because law firms can expect to extract so much value from young employees who blossom, they have a big incentive to try to hire the best candidates right after they graduate.