Why does selling my data on-chain not stop the buyer from using it indefinitely?

On-chain data marketplaces tokenize health, behavioral, and financial data as smart-contract-governed assets. But data is non-rival: selling a copy does not prevent the seller from keeping the original or the buyer from re-sharing it indefinitely. A smart contract can govern the initial transfer but cannot enforce downstream use restrictions, time limits, or purpose limitations once the data leaves the chain. Ocean Protocol's own documentation acknowledges that its Data NFTs represent intellectual property on-chain but are not enforced by legal contracts. Trusted execution environments and Compute-to-Data approaches offer partial answers but neither solves use-limit enforcement once data is decrypted and consumed by a buyer.