Never trust anything that can think for itself….

I definitely get the sense that Mr. Weasley said this to Ginny in the same tone that any Muggle parent would use to lecture a child who had returned home after being abducted.  "Don't talk to strangers!  I've told you a million times!"  However, I also get the sense that this particular warning came across as paranoia to his children, since he works in the Misuse of Muggle Artifacts Office at the Ministry.  He has an abnormal amount of experience with objects that have been bewitched to do cruel and unusual things to their users.  As a result of Muggle-baiting, he often encounters objects such as biting teakettles and regurgitating toilets.  Being Pure-bloods with no antipathy toward Muggles or Muggle-borns, the Weasley children likely have little to no firsthand experience with this type of object.  Consequently, Ginny does not think twice about pouring her soul out to a diary that can respond to her, and Fred and George feel perfectly at ease using the Marauder's Map, despite its seemingly sentient powers.

Although the Weasleys are underprivileged in many ways, due to financial hardships, they are certainly privileged in that they are not subject to blood status bias.  They are considered "blood traitors" by many, such as Mrs. Walburga Black, the portrait of whom screams at the members of the Order of the Phoenix, who occupy her home at Twelve Grimmauld Place.  However, when the Muggle-born Registration Commission interrogates Muggle-borns on the source of their magic, the Weasleys are still relatively safe, due to their definitive place in the Pure-Blood Directory as one of The Sacred Twenty-Eight.  They are endangered due to their continued association with Harry, Dumbledore, and the Order of the Phoenix, but before Voldemort returns to power, they do not expect to be targeted.  Thus, when Lucius Malfoy plants the diary on Ginny in Flourish and Blotts, she trusts that it is merely a really cool diary.

 "My sister used to commit her innermost thoughts to her diary. Her great fear was that someone would read it. That's how the idea came to me of a diary that is itself against you. You would be confiding everything to pages that aren't inanimate."

-J.K. Rowling, Sunday Morning Herald

Ginny does not consider, until it is much too late, that the diary might be the sort of object her father has warned her against.  Fred and George were fortunate that the Marauder's Map did not betray their trust--that the Marauders were the harmless sort of inventors, as opposed to Tom Marvolo Riddle, whose inventions were generally Horcruxes like the diary.  When Harry receives the Marauder's Map from them, he is right to proceed with caution.  Having been Voldemort's target more than once, he has good instincts, and no illusions about his safety.

--AK