Hi everyone. I just noticed this behaviour today, and while I too don't like how I can only open one video at a time, let me give you a little example of why this is a GOOD thing, at least Apple thinks so.
I work at a broadcast school that shoots on Panasonics that record to AVCHD. We use Final Cut Pro 7, so my students NEED to Log and Transfer from the original folder structure, or L&T won't work. Every now and then I get a student who copies just the Clips folder off the card, erases their SD card, and wonders why FCP can't L&T just the MTS files. Heck, just two days ago I had an assistant instructor give a student wrong information about working with their SD card.
This is textbook Apple protecting us from ourselves. Just like with the iTunes, iPhoto, and FCP X and now OS X User libraries, all the contents are still there, just hidden a level so that the people who don't know what they're doing, aka my students, don't mess things up. If our iMacs were on ML, my students would ONLY be able to copy everything, so no more only MTS files.
I'm not saying this is right by Apple, I would be angry too if I had a workflow that got messed up, but hopefully this adds a little context to why Apple probably made this change.