You'll note that Bellatrix hexes Harry in front of Hagrid's hut and Snape stops her: "No! He belongs to the Dark Lord!" Voldemort had an established history of ordering his underlings not to interfere when it came to bringing harm to Harry -- he wanted to do it all himself. I should say that this changes in Deathly Hallows, at which point Voldemort is willing to accept help in capturing Harry.
Was it too much of a risk? Actually, I'd say it wasn't at that point. In fact, Harry left himself quite vulnerable to attack or abduction by chasing after Snape and the other Death Eaters -- he no longer had Dumbledore to swoop in and come to his rescue and Snape could not blow his cover as a double agent. If Snape had protected Harry any further than telling Bellatrix to stop hexing Harry, it would have caused suspicion, especially on the part of Bellatrix, who we know was doubtful of Snape's loyalty to Voldemort from the beginning of the film.
It was also a plot point. Harry couldn't disappear at the end of Half-Blood Prince because Deathly Hallows couldn't then occur as it did.