This isn't a bug as such, more of a possible oversight.

In 14w03 tools must be given CanDestroy to break a block, awesome!

But what about blocks that do not require tools to break? Grass, flowers, most importantly crops of any kind can not be broken in any way in Adventure mode.

Hoes no longer work on grass blocks to till the soil.

Shears can no longer be used to obtain grass or leaves.

Possible solutions:
Allow shears to be able to cut grass, flowers etc.
allow grass to be placed in the crafting slot to get seeds - this doesn't help flowers and crops however.

Add a gamerule that will allow the breaking of blocks that require no tool.

Additionally some adventure mode maps may want the ability to place blocks - a gamerule to allow the placement of blocks in adventure mode.