Why would people launch them?
Marketing, attracting other investors and giving their project attention that they want.
If there is no airdrop then they have to pay for other advertising cost.
But if the project doesn't gathered that much fund during the ICO then that airdrop is worthless.
Devs do airdrops before/after ICO? I'd guess ICO investors are not going to be happy about that as airdropped tokens would be dumped below ICO price as soon as the token is listed on an exchange.
If there is a dump that will happen it will surely not that significant because airdrops normally has so many participants and the share they will receive from the allocated budget on airdrop will be too small that no one can control the price in the exchange.
Not only that, but smart users would hold onto the coin for a little bit just to make significant money, instead of selling 1000 shares at say, 50 cents a share.
I participated in one ICO, and although they did not do an airdorp, the price has already allowed me to over triple my money in the few weeks since the launch. There's significant gains to holding onto coins after an ICO is complete and you can exchange the tokens, instead of selling immediately.