Because it's needed to make the "10 minutes ago" recent activity stuff work. This is different from how it was back when it was breaking the tests.

This failed the tests:

    config.time_zone = 'Central Time (US & Canada)'
    config.active_record.default_timezone = 'UTC'

Then you did this, which passed the tests:

    config.time_zone = 'UTC'
    config.active_record.default_timezone = 'UTC'

... but it didn't give correct "N minutes ago" times, instead giving eg. "11 hours ago" if you looked at it from where I was.  Now it says:
 	
    config.active_record.default_timezone = 'UTC'
    config.active_record.default_timezone = :local

... which both pasts the tests (even in non-UTC places like where I am) *and* displays "X mins ago" correctly.