mxcl/homebrew — comment on commit a633812f95980ec86d3b9e0547470fa5b8eec469
File:  line 0
By mxcl:

We prevent it for install and link too, though we only prevent it if the brew executable itself is not root owned.

There are reasons why users may want to have various brew commands run as root, at least, `brew --prefix` etc. should work if you happen to be root. So we only add this check for cases where the result would lead to broken behaviour.