The reasons for not wading/riding/swimming through floodwater are many.
The manhole/drain/services covers can easily be shifted by the flood water leaving deep holes you'd be unaware of till you step/rode into.
Objects moving unseen in the waters flow could entangle/trip/trap feet.
The waste water system will be mixed in with the water you're riding in/walking in (albeit diluted) giving an increased risk of infection from waterborne diseases. leptosporosis (sp) is one.
Why would you place yourself at risk when the emergency services are already hard pressed to do their job? potentially taking them away from helping elsewhere. And this is not a 'i know the risks etc' moment as if you did you wouldn't go wading around in the water unless you absolutely had to.
I'm not chastising the pictured rider or st for posting the picture i just find it surprising that people are so blase about the risk they've put themselves in.
Right swiftwater rescue trained FF hat off