This is to avoid having some <<< or some >>> in that file if there are conflicts.
If there is a conflict, I just want a message telling me there is a conflict and bazaar should not mess with the file.
With subversion, you can modify the svn:mime-type property. But I don't know if Bazaar have this feature.
The reference says that there is no explicit way of telling so far:
Bazaar currently relies of [sic] content analysis to detect binary files for commands like diff. In the future, a
binary = true
rule may be added but it is not supported yet.