Blizzard goes after the sellers of gold.
They do not seem to go after the buyers.

Two of my real life friends, out of seven or eight of us that still play wow, regularly purchase gold from a site. They have never received a suspension or such, or even a warning. One of the two, was dropping $100 a check for over a year, you would think Blizzard would be aware of the gold transaction.

It is just like with the police.
If they solely prosecute the prostitutes, that is one approach.
But if they go after the "John" as well, that changes things a bit.

I bet if either of my friends (or other gold buyers in general) received an email letting them know the gold they had purchased was confiscated and that their accounts were suspended for 30 days for buying gold. And that next time they bought gold Blizzard would permanently ban the accounts... it might put a dampener on the whole thing.