I found that some wordpress themes and plugins uses activation key approach, to sell GPL licensed theme. Is it illegal to remove the code logic for licensing, and redistributing them?
No, it is completely lawful, and in my opinion a public service.
The only possible argument against it would be that you're circumventing a technological protection measure, a practice which signatories to certain WIPO treaties are obliged to forbid. GPLv3 is very clear that that doesn't apply, in s3:
When you convey a covered work, you waive any legal power to forbid circumvention of technological measures to the extent such circumvention is effected by exercising rights under this License with respect to the covered work