Why Malfoy smuggled Riddle's diary

'A clever plan. Because if Harry here and his friend Ron hadn't discovered this book, why - Ginny Weasley might have taken all the blame. No one would ever have been able to prove she hadn't acted of her own free will. And imagine what might have happened then. The Weasleys are one of our most prominent pure-blood families. Imagine the effect on Arthur Weasley and his Muggle Protection Act, if his own daughter was discovered attacking and killing Muggle-borns. Vert fortunate the diary was discovered, and Riddles's memories wiped from it. Who knows what the consequences might have been otherwise...'
'Don't you want to know how Ginny hot hold of that diary, Mr Malfoy?' said Harry. 'Because you gave it to her. In Flourish and Botts, you picked up her old Transfiguration book, and slipped the diary inside it, didn't you?'
'Prove it,'
'Oh, no one will be able to do that,' said Dumbledore. 'Not now Riddle has vanished from he book. On the other hand, I would advise you, Lucius, not to go giving out any more of Lord Voldemort's old school things. If any more of them find their way into innocent hands, I think Arthur Weasley, for one, will make sure they are traced back to you...'

DOBBY'S REWARD - Pg 355

Lucius Malfoy had been sacked as a school governor.