I think it's because the Calendar Block is just a window to the calendar functionality. No events can be added/deleted via the block itself, but via the calendar functionality, which is a course/site/user level context instead of a block level context.
There was a further confusing thing about the "teacher to the block" assignment: the student got a "Turn editing on" button for the course, but couldn't turn the editing mode anyway. I didn't file a bug yet as I'm not using the very latest Moodle 1.8+, I'll upgrade when I get a chance. I thought that the editing mode would give the particular student options to move/hide/delete the calendar block.
Roles and permissions
Teacher role for course student in calendar
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