Actually, the PSL did have the safety sear, as did the Yugo M76 (and the British L1A1). These were included in the design because there was no reason not to and so assembly was easier.
The M76's that came in several years ago all had the safety sear still in place. Folks paid quite alot for those rifles only to have the ATF show up at their door and confiscate them. Caused all sorts of bitter anguish and anger. These M76's you see for sale in various places in Shotgun News are the remnants of those original rifles. They were taken down, the receivers destroyed and replaced by a US made (Ohio Ordinance I think) one and reassembled. Therefore, those on the market currently (save for that small batch imported in the 1980's) are just parts kits.
The PSL's that got recalled were made from original PSL receivers with the original information ground off the receiver just forward of the magazine on the bottom. You can look there and see the buffing/grinding marks. They then had the safety sear welded up and cleaned up. There is no reason this could not be allowed because the PSL (like the M76) were never machine guns, so once a machine gun, always a machine gun should not have applied. They were never machine guns and in the case of the PSL's, were not easily convertible as the same holes that would have to be drilled out in the originally welded-up rifles as would any AK receiver.
The British L1A1 was also built in this manner after they decided that full auto was worthless. Even though most L1A1's were never machine guns, were never fully-auto, the fact that they had the safety sear means we cannot import original L1A1's (like, say, original SKS's FN-49's, or MAS 49/56's).
Lunacy.
However, virtually all US semi-auto AK's are built without the safety sear. Whether they are M76's, various 7.62x39 AK's, PSL's, VEPR's, or Siaga's, they all lack the safety sear. And while mostly the PSL's use the 7.62x54R cartridge, there have been no other reports of trouble on this front. That is not to say that it cannot be the problem, and the lack of evidence need not necessarily be proof of no problem, it is a strong indicator that something more specific was defective in that specific rifle like a tight chamber or some such thing.
Ash