Believe it or not, there is a good reason. Whether or not the manufacturer is covering their behind or yours is up for debate.
There is a big difference in the way a document safe is designed when compared to a gun safe. The insulation in a document safe, especially the inexpensive imports, contains a lot of moisture. This moisture is designed to steam the inside of the safe in the event of a fire. Gun safes use insulations that contain much less moisture (typically drywall).
This insulation is so wet, that it can even rust through the steel lining of the safe. In many cases, you will find that these "safes" are built out of more plastic than steel.
You can not control this moisture. No dry rod or silica gel will work, as it is working against the safe's natural design. Trying to "dry out" a document safe of this type of insulation is similar to trying to dry out a swimming pool full of water.
These safes are also not CA DOJ approved for firearms storage due to their lack of security. As mentioned above, most inexpensive document safes are not much more than a fire box with a privacy latch.
Buying security devices is something that should be done at a place that specializes in such devices. These things can be explained prior to purchase.
I have one. It has a label etched into it that specifically says, "Caution: Not for storage of flammable materials."
Well ***. If it's not flammable I wouldn't need to store it in a fire safe now would I?
That is funny.
I would assume they mean flammable items that could be explosive if heated. They make special containers for the storage of those items as well.