Erik Voeten points to a paper (pdf) by James Hollyer and Peter Rosendorff arguing that authoritarian regimes tend to ratify the Convention Against Torture with the express purpose of torturing its citizens anyway:
And it is important to those signatories that all observers understand that they have no intention of complying at the time of accession. The logic, while counterintuitive, is straightforward: an elite facing threats from a domestic opposition can mitigate these threats by engaging in torture. If there is any additional cost to the elite of signing and then being found to torture, the act of signing the agreement signals to the opposition the strength of the elite’s commitment to remaining in power.
DiA digests the findings:
[F]orcing someone to admit to something he might have done does not send a strong signal of power. Forcing someone to confess to a crime that everyone knows he could not possibly have committed, on the other hand, is terrifying. Similarly, a regime that tortures its opponents and refuses to sign the Convention Against Torture shows that it fears international opprobrium. A regime that tortures its opponents and blithely signs the Convention Against Torture anyway shows that it fears nothing.
Messrs Hollyer and Rosendorff believe the intent is to show how dedicated the regime is to maintaining power, how much it will sacrifice. But there is another possible signal: the regime shows its opponents that it knows international pressure cannot disturb its grip on power in the slightest.
Reminds you of someone, doesn't it?