Why shouldn't you squash a wasp after it stings you?

There is no reason to not squish it after being stung. To tell you the truth, wasps are some of the very few pest insects that do not benifit us or the environment very much. The only reason why you shouldn't is if you have serious allergic reactions to stings, which nutralizing the poison should then by your first proirity. But, if a bee stings you, there is really no need to squash them, since they die after stinging beings anyways.

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I have to disagree with some of the above. Wasps do, indeed, benefit us. In spring, queen wasps feed on nectar and as such are pollinators. Once they have larvae wasps have to capture other insects, their grubs or caterpillers for their larvae to feed on. In this way they remove a lot of what would otherwise be pests from the garden and are truly a gardener's friend.
Wasp larvae excrete a sugary fluid as they feed, and the adult wasps feed off this. The only time we really notice wasps, short of being near a wasp nest, is towards the end of the year. Production of larvae drops then ceases altogether, so the adult wasps lose their primary food source so have to go looking for other sources such as ripe fruit and other sweet liquids. This is when we come into contact with them.
Most of the time wasps are doing a very useful job. We really shouldn't squash them.