What they might mean by that is that the machines have data on them that aren't already on other computers, it's not always that they want the data secret or private, as much as that they want to preserve some old photos, ripped CDs, school papers or whatever it was the machines were used for.

In terms of getting data off of old machines, my policy has always been that an investigtion is fine as long as I don't look for too long. I did actually at one point find myself reading old school papers that were on an SE I had gotten from a GoodWill tore, and more than anything, it was weird, like the machine had been used in a creative writing class.

In terms of getting software off of old machines, my recommendation is personally to restrict it to drivers, and EXTREMELY rare pieces of software that are nowhere else on the internet.

The simple fact of the matter is that it's already archived somewhere else, and the other archiving group has likely done a better job at capturing the original data that was on the installer disks, not data that was in somebody's working installation of an OS. When you make a bad copy of a piece of software, you make it more difficult to find a good copy.