(1) [Code of 1.8]
These error have been fixed in revison 16195 only for 1.9.1
1.8 branch still contains these errors. See rev 16195 of CUserMacro.php to know how to fix if for 1.8

(2) [Documentation]
How it works in 2.0:
Request ("method":"usermacro.get"):

{ "output":"extend", "templateids":["10124"] }

Response:
{
"jsonrpc":"2.0",
"result":[{
"templates":[

{ "templateid":"10124" }

],
"hosts":[

{ "hostid":"10126" }

],
"hostmacroid":"52",
"hostid":"10126",
"macro":"{$MACRO6}",
"value":"foo-host"
}],
"id":0
}

where:
10124 - ID of a template
10126 - ID of a host which linked to the template and the host has a macro {$MACRO6}

Generated SQL:
SELECT DISTINCT hm.*,ht.templateid FROM hostmacro hm,hosts_templates ht
WHERE
(ht.templateid IN ('10124')) AND hm.hostid=ht.hostid

I do not see much sense to have such feature (IMO) and definitely its description in documentation is BAD.

So, consider to:
a) remove this parameter from API and doc;
or
b) Write good description in documentation:
http://www.zabbix.com/documentation/1.8/api/usermacro/get
http://www.zabbix.com/documentation/2.0/manual/appendix/api/usermacro/get

My suggestion is: "Select all host macros from hosts which linked to given templates."

Current description "Template IDs" it very similar to a "Host IDs" description for a hostids parameter, which can be interpreted as "select macros from templates" but actually its meaning is absolutely different.

How users can guess what "templateids" means ?
They have to look to source code? or generated SQL? - is NOT acceptable for users!