Yes, It is true and Garmin is quite explicit about what you can and can't do with Maps delivered on SD Card. You are also correct, in your observation that BaseCamp will read the Maps from the SD Card if connected via any SD Card Reader (your GPS is one of those). It does not "copy" the maps and then erase them. It reads them and creates an "index" of the Map and saves that index so that the next time you plug in the SD Card with that Map it doesn't have to re-index the Map - taking less time.
Sent on the road in Salta, Argentina...