Actually it's a bug, when mining redstone with an iron pickaxe it doesn't drop anything (pocket edition)

Duplicate of MCPE-116, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.

funny, they should add that feature. making redstone ore drop redstone so we can get ready for future updates. as of right now, its not a bug

Duplicate of MCPE-116, this will probably be fixed- or should I say added- in MCPE 0.8.0 alpha.