Its a Sprint product...and the customer returned to a Sprint store. Legally,...they returned it. Period.
But of course...its always recommended to return they Mb way Sprint instructs u too. Just to avoid any potential headaches.
A contact of mine in FL went into Sprint corporate store while on his way to the airport, returned MB and closed his Sprint account..... he was moving to Australia. They gave him documents that said he did so. Been 3 months now. He says he has seen no issues from that.
Edit: It was a company/business account...not normal personal account, so maybe that had something to do with it.