When on gamemode 2 (adventure) you can place blocks
MC-12084
You can place blocks on adventure mode
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0
Vladimir
(Unassigned)
Unconfirmed
block
Minecraft 1.5
Retrieved
This is intended.
The blocks which can be mined by the player (correct tools !) can also be placed, see http://www.minecraftwiki.net/wiki/Adventure_Mode