Apparently getting their flying insurance is mandatory at around $220. Sounded pretty steep to me for less than a week's flying.
Sounds like the price you would pay for a stdnard commercially available annual renters policy with $5000 coverage for the hull deductible. Insurance is not mandatory in the US and any insurance the FBO holds will probably be there to protect them and not you. So it sounds like their deductable is $5k and if you bend the plane they will come after you to make good.
You need to know about this before you rent and at least make an informed decision on whether to cover yourself or not. All the standard travel insurance policies I have seen in the UK exclude any flying if you are not a passenger, so you are probably also not covered for medical stuff if you get hurt. The good news is an American hospital will treat you in the case of an emergency, the bad news is they will agressively pursue you for the cash and have no moral problems in forcing you into bancruptcy if you don't want to pay it.