AEGY filed a Form 15 and Deregistered their stock and became a non-SEC filer SKTO was already a non-filer. So the SEC can't revoke the ticker. So they remain on the greys until FINRA delists them - and it can take many years.
In the case of a company like HJOE - they were suspended and after the suspension they tried to file a Form 15 to avoid being revoked but it was after the suspension so it didn't work and their ticker was revoked within a few weeks after the 10 day suspension.
This is bad for shareholders if they want to write off the loss they have to call their broker and make arrangements.