Using the Select All command on masks, selects all masks, not all unlocked masks. Here's how to replicate it:
Result: Mask 1 will also move and create a keyframe on it's mask path even though it's locked.
You don't need to make keyframes or more than 2 masks to display the bug, but it helps show the full brunt of the problem. The fact that not only can a locked mask be moved, but that keyframes for that mask can be created or overwritten, makes Rotoscoping a potential nightmare (on top of the nightmare that is already is.) I discovered this bug about 2 weeks after the launch of CS4 and I've filled out numerous bug reports and feature requests over the past 2 and a half years. I'm very displeased that it wasn't fixed in CS5 or CS5.5. I've replicated it on every Mac and PC that I've tested and I haven't found one yet that didn't do it. CS5.5 (trial) has only been tested on the Mac so far. If anyone else can test and confirm, it would be greatly appreciated.
Yes, it is there. I wasn't even aware that this could be an issue, since I always select my masks visually or directly in the timeline, but you have a point.