You're wrong. There are at least three full-featured FOSS GSM stacks: Qtopia (discontinued by Nokia, but forked by Openmoko community as QtMoko), freesmartphone.org (with two iterations - first one written in Python, and second one written in Vala; I'm using it on my phone) and oFono.
You can't use non-certified firmware for modems, like OsmocomBB for TI Calypso, on public networks (yes, there is even free software GSM modem firmware!), but GSM stacks in user space are possible and legal.
You can't use non-certified firmware for modems, like OsmocomBB for TI Calypso, on public networks (yes, there is even free software GSM modem firmware!), but GSM stacks in user space are possible and legal.