As vedmak mentioned here ZBX-5047, host.update can't update usermacros.
But it is clearly written in documentation, that it is possible:
https://www.zabbix.com/documentation/2.2/manual/api/reference/host/update "Updating host macros"
https://www.zabbix.com/documentation/2.0/manual/appendix/api/host/update "Updating host macros"

And even in the page with changes in 1.8/2.0:
http://www.zabbix.com/documentation/2.0/manual/appendix/api/changes_1.8_-_2.0 "To update host macros host.update can be sued."

If it is bug in documentation, please correct it.

Thank you.