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Suppose in local time (which is, say, UTC+3) the DST transition is on something like "last Sunday of 10th month at 02:00", which might then in UTC become "last Saturday of 10th month at 23:00, except if that Saturday is the last day of the month, then the second to last Saturday". In effect, if conversion to UTC causes the transition to move to a different day, it might also be on a different week or month, causing headache.

(Or did you ask something else entirely? It wouldn't be the first time I misunderstood things about time zones.)

I bet it's North America bias—we roll our time changes across the continent one TZ at a time, at 2am local time (which also explains why the default is "/2").

Perhaps to decouple DST transition rules from other time-zone changes. So if the time-zone offset is adjusted for some other reason then the DST rule does not also need to be changed.