I bought Samsung Galaxy S3 recently, plugged it in via USB and was not able to browse it because, as far as I understood, MacOSX has no support for "media devices" whatever that is. I had to download some obscure Samsung "Kies" software through which I was able to get to the filesystem and upload some files. Wasted hours.
Starting with Honeycomb the storage partition is exposed through MTP (or PTP), this way it can be accessed concurrently by both Android and your computer. Before Honeycomb, said partition could be accessed as an USB mass-storage drive but that required it to be unmounted, thus making all data unavailable to Android while it was connected to a computer.
In my experience only Windows 7 has passable MTP support. On Linux I had mixed results with gvfs and mtpfs (slow and crashy). jmtpfs [1] is a nice replacement, though. Google also released an application for OS X [2] but I can't try it since I don't own an Apple computer.
My solution to the problem was to install a WebDAV server (such as [3]) on my device (Galaxy Nexus) and access it wirelessly.
Why can't Apple support MTP? Linux gets cited for lack of hardware or protocol support (MTP to an Android device works fine in Linux, btw), but it's not Apple's fault when they can't or don't provide drivers for common, standardized consumer electronics?
Even though my Samsung phone is seen as a USB device (and it works on MAC OS X), but maybe in the newer models they removed this functionality (and called it a feature)
People may complain that iPhones need iTunes but then again it's iTunes not the gigantic pile of crap that is Kies
It was pure engineering tradeoff to improve things in the long term. If anything OEMs would have preferred the old 'just works', sub-optimal, gives-an-excuse-to-obsolete-a-phone, lower support issues, solution. The ball is now in operating systems' court to support this standard in a way it wasn't envisioned to work a few years ago.
OS X has perfectly find support for media devices, and has worked fine with every digital camera, SD Card or other card reader, etc, that I've attached via USB in the past decade!