This wouldn't really prevent them from profiting - an unsuspecting user could still pay the ransom, and then never receive a decryption key, so would be both out of the money and lose their data.
Sure, it wouldn't prevent it completely, immediately. But 1) many users will do a search beforehand to see whether paying actually works. The less often it has worked for others, the less likely they will be to pay, and more importantly, 3) it would prevent them distributing new versions of the malware, which would prevent them profiting once antivirus caught up to the existing versions.
