Why does one type of tethering obstruct MTP while another doesn't?

When I use the native USB tethering function of my phone, it makes MTP stop working. I can't use both MTP and tether both at the same time. However, when I USB tether via ClockworkMod Tether , I can still use MTP even while tethering. What is the technical reason for this difference? If it's technically possible to tether and use MTP at the same time, why is that not typically enabled in the native functionality of the phone? I use OS X, so by MTP I mean Android File Transfer. But I think this works the same way on Windows and Linux too.