It is not a great idea to have the float bowl gasket seal against pressure, because then you wouldn't know the carburetor was flooding. Fuel would go through the main jet and the discharge nozzle into the engine's intake pipe, without any external sign of trouble. That would be bad: if you left the fuel on when you parked the mower, it could hydraulic the engine. Chances are the starter motor wouldn't have enough torque to break the connecting rod when you next tried to start it, but it would be quite annoying if it did, and B&S rods are not very strong.
Warning: another of my anecdotes. I once saw a qualified motor mechanic trying to fit a K-Jetronic fuel injection system to a V8 Holden in place of the carburetor. He didn't bleed the fuel line properly, so when he switched on the ignition the system didn't reach full pressure, so fuel sprayed out of an injector, into the intake manifold. Then when he tried to start the engine, it was hydraulicly locked by petrol in the cylinder. That wasn't a big problem - car engines have strong connecting rods - but he needed to get the petrol out of the cylinder. He did the obvious thing, and removed the spark plugs, then cranked it. Unfortunately he had not disconnected the distributor low tension lead, so some of the loose spark plug leads sparked to ground while half a litre of petrol squirted violently out of a spark plug hole. The ensuing fire was unhelpful to the general condition of the car. It would have been seriously nasty if anybody had his head under the bonnet at the time.
