As the docs point out, that's the way the API is expecting that parameter.
send function from the
Mandrill_Messages class has this in the docblock:
* - global_merge_vars array global merge variables to use for all recipients. You can override these per recipient. * - global_merge_vars struct a single global merge variable * - name string the global merge variable's name. Merge variable names are case-insensitive and may not start with _ * - content mixed the global merge variable's content
Maybe write an email to Mandrill, although i doubt they'll consider any changes to their API.