hplip installs hp-setup to configure printers This involves generating a .ppd file and writing it to /usr/local/etc/cups/ppd This can be done by a non-root user. The directory written: $ ls -dl /usr/local/etc/cups/ppd drwxr-xr-x 2 root cups 512 Mar 28 12:09 /usr/local/etc/cups/ppd The user involved: cupsadmin $ grep cupsadmin /etc/group wheel:*:0:root,cupsadmin operator:*:5:root,cupsadmin cups:*:193:cupsadmin Note that while user cupsadmin is a member of wheel, the directory written is only writeable by root; and while the user cupsadmin is also a member of cups, the directory is not writeable by group cups. The file written: $ ls -lt /usr/local/etc/cups/ppd/HP* -rw-r----- 1 root cups 31122 Mar 28 12:03 /usr/local/etc/cups/ppd/HP_Officejet_Pro_8500_A909g.ppd
hp-setup talks to cupsd to add the printer and it's cupsd that generates the ppd file.