Another point to consider...in the pre-1965-ish period of time, it might be safe to say that 50% or more of the U.S., (geographically speaking), people didn't always remove keys from the ignition at all.
G.M. Actually offered in many models of cars a "keyless" ignition option that the driver had to choose to lock if they wished to.
Obviously, this presented another whole problem with Baby-Boomers of the day and vehicle theft of the mid-60's and beyond brought about changes including locking columns and locking shifters beginning to become standard equipment in 1969 &1970 for passenger cars and pickups only a few years later.
Changing times, we are once again back to "keyless" ignitions but a whole lot smarter cars with technology.
All to say, a power top, being optional equipment anyways, it wouldn't be any sort of security issue in the day those cars were engineered.