While it might seem the deductible is only $250, the claim could cost much more in the long run. With most policies when you file a claim your premium also increases for a number of years as well as your company is now seen as a liability. It is not uncommon for the deductible and an increase in premiums over years to cost more than the actual claim.
Like someone mentioned above, your insurance company will usually fight to protect their interest. While they may initially have you pay the deductible, they may choose to go after the other company and their insurance to get reimbursed. If your insurance wins, then your claim will be removed and your deductible reimbursed. This is often the case when we are talking a large claim for a vehicle, personal injury, or property damage. But how hard is hour insurance going to fight for the relatively small claim of a lawn mower?
Also I didn't see it described above, but what were the circumstances of the mower being stolen? Was the building actually broken into at night? Or was the mower left in an outside locked pen at night. Was it stolen during the night while they were closed or during the day? The reason I ask is that back when I was in high school I had a stereo stolen in the middle of the day while my truck was at the mechanic. The gate to the pen was shut but not locked as they were open, some one walked into the pen and stole a couple radios. the shop was liable as they did not properly secure our materials by keeping the gate locked during the day. Had the thief jumped a locked gate then the mechanic would not have been liable.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!