It's just that Virtualbox requires a nonfree compiler to build, so it won't be in Trisquel 7.
Actually, the GNU/Linux version is free software:
http://changelogs.ubuntu.com/changelogs/pool/multiverse/v/virtualbox-guest-source/virtualbox-guest-source_4.2.10-dfsg-0ubuntu2/copyright
- [Trisquel-users] Trisquel in Virtualbox = no audio dgkdn2x+1k541o
- Re: [Trisquel-users] Trisquel in Virtualbox = no au... dgkdn2x+1k541o
- Re: [Trisquel-users] Trisquel in Virtualbox = no au... mikko . viinamaki
- Re: [Trisquel-users] Trisquel in Virtualbox = no au... gustavo_cm
- [Trisquel-users] Re : Trisquel in Virtualbox = ... magicbanana
- Re: [Trisquel-users] Trisquel in Virtualbox... legimet . calc