I think the why it is done the other way is that the 3" shell can cause the magazine to move because of recoil and if the spring is too light, then it does not control the shell in the mag, so that when it comes time to load the shell in the mag that the shell is out of position because the spring didn't keep it against the shell stop in position. Think of what the recoil would do if there was no spring in the mag. Because of inertia, the shell would slide forward in the mag tube as the recoil hit and take it out of position for feeding. The spring has to be strong enough to hold it against the shell stop despite recoil. A weak spring will let it move out of position.
Hopefully I worded this correctly to understand.