ReasonFree isn't too bad IMO but everything has room for improvement; I would imagine they watch for modifications on bait files placed around the system as opposed to strictly monitoring for file extension changes alone, that'd be pretty "suck-ish" alone IMO. As for signatures... Very unreliable, even for Petya.
When you're up against Petya you need to set hooks to block a handle opening to the physical disk, or block write attempts to it. Alternatively, work with a device driver to restrict access to the Master Boot Record. Even if Petya executes, if the MBR access is blocked off for write then it cannot infect the system with it's pay-load. Take this advice as an actual method which I have used myself personally, it works.
They could monitor API calls but it would be tricky for ransomware BB monitoring since there are tons of third-party libraries out there for file encryption, not all work via the CryptoAPI. Plus the functions from the libraries can be statically linked as opposed to being used through the usage of Dynamic Link Libraries.
A good idea for them to use, if not already, would be a file system mini-filter device driver, and then watch for the file modification attempts; through this they can attempt to identify encryption attempts and then call PsGetCurrentProcessId or IoGetCurrentProcess for analysis purposes.
