Multi-Erase Deleting Stamps Inside Locked Groups

Difficult to explain - the multiple placement tool has a feature where holding "Alt" turns it into an eraser for a certain "type" of stamp. For example I have a cluster of Zen Garden Rocks in a locked group, and would expect those stamps to be "protected" from erasure (by virtue of the fact that they are locked) but the erase tool erases them anyway. Stamps in groups, especially protected groups, should not be affected by Alt+ erase feature of the multi-placement tool.