the booster stage has a fairly high ballistic coefficient, even when it is empty of propellants - as i said, the casing is very strongly-built, and quite heavy. it's perfectly possible, especially since we're still short of icbm reentry velocities, for the reentering booster stage to "outrun" the heating. that is, because it reenters at high speed, and because it's pretty streamlined, it doesn't have time to heat up substantially before hitting the ground, and this is without it having the sort of thermal protection a reentry vehicle has. there might actually be some ablative material on the lateral surfaces to protect against heating on ascent, and the casing of the motor has a good deal of thermal mass. the satellites you're thinking of break up at relatively high altitude because they're not very strongly constructed - this is part of why launch vehicles don't jettison their fairings on ascent until after climbing above a certain altitude. it doesn't take much to get them to break up - even very low dynamic pressures will do it, especially since components like solar panel wings are very fragile.
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