If you don’t cause the carbon to be captured, it’s not a credit. If you protect existing trees, no new carbon gets captured that wouldn’t otherwise. Plant the trees on land that you cannot lease/sell for logging. THAT’S a credit.
I don’t earn $500 by not buying the LEGO THE LORD OF THE RINGS: RIVENDELL™, I simply didn’t spend it. See how a credit is different from stopping a loss?