It doesn't stop the infection as such, what it seems to do if Petya detects (as it looks like it searches for it) perfc.dat is stop it from running the file scrambling portion of the code on your machine. So while you still have access to your machine and all your data you are still infected.
So what does Petya do if perf.dat is detected, if not the file scrambling? What it does is use your computer as a foothold into a network to continue spreading internally (not externally, unlike WannaCry).
From what I found while researching this answer, the only thing the perfc.dat 'vaccine' stops is the file scrambling, I believe (In that I couldn't find any source that said otherwise) that it still attempts to lift credentials from RAM to allow it to gain admin access, and travel further through a network.
As to whether its a bug or a feature, the f-secure link I've posted at the bottom has a nice analysis suggesting that the malware, which has a very sophisticated network propagation component, and less sophisticated MBR and user mode component (this does the encryption/decryption of files, and apparenytly does work properly), suffered a suddenly reduced deadline after WannaCry flung EternalBlue to headline news, reducing the attackers' potential options for spreading around a network.
I'm not sure if we can be sure, but I'd put it down to the 'vaccine' being a bug, rather than an intended feature. F-secure did some very nice write ups about Petya on their 'From the Labs' blog, most of which can be found from the one I linked below, I'd link a few more but I don't have the reputation.