It's not dumb as a brick. There are 1 Int creatures, and even they have some intelligence. It's a standard animal intelligence - it can toss stuff at you if you annoy it, just like any big animal can do.
Here, Rhinos have a 2 Int in the MM, and here is a real life one tossing a warthog:
[video=youtube_share;M8yN1sNxWg8]http://youtu.be/M8yN1sNxWg8[/video]
It's assumed the DM is capable of using the general rules for all things, monsters and PCs. Improvised weapons are part of the rules. So are the shove and grapple action for instance, neither of which are in most monster writeups either. You're not a slave to the write-up, since particularly in this edition the authors give the DM a range of additional tools which are used where appropriate for any encounter.
I would actually, but I am replying to someone who says he goes strictly by the write-up, and strictly by the write-up he can't. And if he is flexible enough to expand beyond the write-up for dropping alchemist fire, then it's fair game to use the improvised ranged attack in response. You don't get it both ways - you can't apply to strict-write-up when it benefits your position, and then flexibility outside the write-up when that pleases you. Either we're strict, or not. He has to pick one.