See T13240. Ref T13242. When we're issuing a query that will raise policy exceptions (i.e., give the user a "You Shall Not Pass" dialog if they can not see objects it loads), don't do space filtering in MySQL: when objects are filtered out in MySQL, we can't distinguish between "bad/invalid ID/object" and "policy filter", so we can't raise a policy exception.
This leads to cases where viewing an object shows "You Shall Not Pass" if you can't see it for any non-Spaces reason, but "404" if the reason is Spaces.
There's no product reason for this, it's just that spacePHID IN (...) is important for non-policy-raising queries (like a list of tasks) to reduce how much application filtering we need to do.