There are four main areas for use of the sin bin
a. Repeated infringements;
b. Deliberate break down of the play, especially in try scoring positions:
c. Cooling off; and
d. Dissent
Thats the quote on sin binning out of the referee's guidelines for 2011 (there is no section on sending people off)
Both fall completely inside the category of "cooling off".
There is also no section in the rule book which states foul play like tripping, high tackles, or headbutts are automatic send offs, nor does it state that you aren't allowed to send someone for the bin for them.
In fact, the only section of the actual rule book which talks about sin binning and sending off players states:
Power to dismiss
6. In the event of misconduct by a player, the Referee
shall, at his discretion, caution, temporarily suspend
for ten minutes, or dismiss the offender