There is no free space monitoring in Monit, only used space monitoring.
There is systems who can easily change disk size (like openvz containers), in them, admin should every time reconfigure monit with new values of space usage, which is tedious (and easy to forget), or code a reconfiguring script, which is complicated and a bit strange requirement.
Also, most human beings prefer to pay attention to free space, not to used space, because lack of free space lead to server failure. To calculate free space from used space earthlings should first determine total disk size (in appropriate units) and then subtract used space from it (in the same units). Which is may be fun, but complicated and error prone.
Used space percentage monitoring does not fit the bill, because in modern times large disks are prevalent, and 1% could be many gigabytes and not yet signify critical condition.