brew services start php70 with unix domain socket - without sudo?
The command (sudo) brew services start php70 (or better (sudo) brew services start homebrew/php/php70) copies a special plist to a distinctive folder and launches and starts it as a launch agent or daemon.

Without the sudo prefix it's copied to the $USER's LaunchAgents folder (~/Library/LaunchAgents/) and started with $USER rights.

With the sudo prefix, the plist is copied to /Library/LaunchDaemons and started with root rights. It will launch several processes (one parent root php70-fpm and several child _www php-fpm processes).

Only the latter will and can create a UNIX socket file with the owner _www.

This is the expected behavior.