I don't believe PA has any "prevailing law". Tenants are responsible for themselves. If they move into a bug free unit, it's their problem if bugs end up in there. I don't understand why some other states can hold the landlord responsible for something that isn't of their own doing.
Example: if I live in a row house (which I don't) and my next door neighbors end up with bugs (roaches or bed bugs) that don't realize they shouldn't come on my side of the wall, there is basically nothing I can do other than rid my own house of the bugs. Again, same thing in a rental - just because I own the building doesn't mean I am going to make things my problem when personal responsibility from adults should prevail. Bed bugs apparently are a part of life in today's world - that doesn't make them a property owner's problem - they are a personal issue.
I believe a few years ago some legislator from Philadelphia tried to get a state wide law passed making it a landlord's responsibility but the rest of the state shut him down quickly - Philadelphia slum problems shouldn't affect the rest of the state and thankfully the rest of the legislators agreed.
by the way, I know my places are bug free when rented because we have a lady with a bed bug sniffing dog in my area and I have her come out and "sniff around" in between tenants. She's been doing this for several years - dogs are certified through some organization.
I think the legal justification for the law that LL is responsible for pest control in MF is because the tenant cannot control what comes under the wall from the neighbors. The LL bears more responsibility because he controls who the neighbors are by his selection of tenants, and because he has more skin in the game when it comes to being pest-free -- tenants are transitory, LLs are forever (at least until they sell!).