IMAP keywords are prefixed with $

The "Add IMAP keyword" filter rule action prefixes the specified keywords with $, so when one configures the keword as `my_tag`, the actual keyword added is `$my_tag`. Why is that, and is it possible to add keywords without that prefix?

Context: having keywords without prefix would help create filter rules that set keywords that map to Thunderbird tags. As it stands, for this to work the Thunderbird tags would have to be prefixed with $.