I can confirm that I experience this same problem, after trying to boot Ubuntu 16.04 from a usb-stick, which I created on an Ubuntu 14.04 system. Pressing TAB and selecting live from the menu doesn't help.
So I think that sudodus is right.
I guess I have to wait for an update for my 14.04 system, before I can use it to create a working usb stick.
Installing from a DVD doesn't help either.
The newest usb-creator-gtk in the repositories is 0.2.56.3, which was probably used when creating my usb stick.
I can confirm that I experience this same problem, after trying to boot Ubuntu 16.04 from a usb-stick, which I created on an Ubuntu 14.04 system. Pressing TAB and selecting live from the menu doesn't help.
So I think that sudodus is right.
I guess I have to wait for an update for my 14.04 system, before I can use it to create a working usb stick.
Installing from a DVD doesn't help either.
The newest usb-creator-gtk in the repositories is 0.2.56.3, which was probably used when creating my usb stick.