This, to me, is the crime. They are purposefully destroying data because they know it poses a legal risk to them. That is different from deleting data that is costing you money and isn't useful to your business. This is also different than counseling them to delete data to avoid problems if there is a security breach. Both of these are valid reasons to delete data in support of the business and in defense of risks associated with holding that data. Counseling that you should delete data because it may be used as evidence against you in a criminal probe though is obstructing foreseeable future investigations because you believe you may be committing crimes. The distinction, I think, is clear.
I agree with you that lawyers should be aggressive about protecting their clients and counseling them but when that counsel is to tell them to obstruct investigations they, to me, are now part of the crime.