Many flies are not health risks. Cluster flies for example. Granted on occasion 20,000 or more will invade a house in a given autumnal day, but they pose no health risk unless you are an earthworm.
Should people not learn to distinguish between flies so that they only kill those that are a health risk?
Reducto ad absurdum - but of course everybody kills. As was mentioned earlier, if their kids have head lice or worms. Now neither is likely to kill you - but we kill them anyway. We kill every time we wash our hands.
Most objections to killing seem to revolve around "I like creature X so others should not kill it, however its okay if I kill creature Y". It may be emotionally satisfying, but it isn't logical.
Cluster flies can be a right pain if they decide to hibernate in your house. I think that's a legitimate reason to swat them. I don't think we're disagreeing here. Just coming at the same line of argument from opposite ends. Like and dislike should play no part in wether end the life of another creature. I don't like house cats.... But believe it or not I don't think that gives me the right to shoot them even if it was legal. However having wasps buzzing round my kids is just plain going to end in trouble so I will take action.