[cperciva]

Attorneys (including defense attorneys), as officers of the court, are ethically barred from lying to the court or bringing witnesses whom they know intend to lie. Defense attorneys will sometimes tell their clients "if you're guilty, don't tell me" in order to avoid such ethical restrictions.

Of course, not all lawyers live up to their ethical obligations and if the only evidence of an ethical lapse is protected by attorney-client privilege, it's unlikely that they would ever get caught.