>Why not just shake them inside the hive regardless of heat or cold? I keep hearing this, what you say, so there must be a good reason, but I need to know the answer.
On a day they are flying they may be defending the hive. Shaking strange bees into a hive may set off a fight. On a day they are not flying they probably aren't trying to defend the hive, and they are more likely to be accepted without a fight.
Shaking them in front of the hive makes them crawl into the hive humbly wanting to be accepted. Dumping them in the hive can set off aggression by both groups.