Although the manufacturers recommend not using the portable gas stoves indoors you can safely use them indoors as they're no more unsafe than a standard propane or gas stove inside your house.
I have an electric stove and have used one of those portable butane stoves inside my house to cook with during a power outage during a cold winter storm and it was very safe.
As long as you use the stove correctly and not keep the Butane or propane canisters near any other heat source or flame then it should be just fine.
Hotels and other places sometimes use those portable stoves indoors without any problems.