This is standard practice. Leaving magazines fully-loaded for long periods of time, especially aluminium-bodied magazines like these, causes the lips to bend out of shape from the upward force of the compressed spring. "Lip creep" leads to a really high rate of double-feed malfunctions. For this reason, the magazines are traditionally stored empty while the ammunition is stored on stripper clips, and the "spoon" adapter makes for very quick work of loading when you actually need to go to war.
Well, Magpul PMAGs are pretty awesome because they come with a snap-on dust cover that pushed the ammunition down, away from the lips, so you can leave them fully loaded indefinitely as long as the cover is on.