When explaining why the trio attacked him in the Shrieking Shack, Snape tells Fudge that Sirius bewitched them into doing his bidding to escape. Was he covering for them or did he genuinely believe his own words? But even if he did, would it really matter to him? Up until then he had been using every excuse he could think of to get Harry expelled and now he has the best excuse he ever had. Why didn't he use this golden opportunity to get what he wanted on account of Harry aiding and abetting the most wanted fugitive in the country?