If picins is not free, why does it exist in MikTeX?
It seems to be due to licensing issues and the package author being unavailable. See the following thread and especially the post in the link: [tex-live] picins is missing from tex-live .
Also, on why it's in MikTeX. On their homepage it doesn't say anything about the license of picins (it's silent about version and maintainer too). So, to connect with a comment above, it seems they are not as strict as TeX Live when it comes to these things.