Zimbabwe's suspension had to do with their government interfering in the board's election to get the members they wanted in power. I can't recall South Africa being banned in recent times unless you mean apartheid? Not sure how both of those are comparable to the present situation with the BCCI, which is a completely private entity (although our government is doing it's best to get it's hands deep into it's coffers).
As for the perceived ban, has there been any official ban on Pakistani cricketers participating these days? I remember one being issued informally in 2009 which resulted in the players pulling out of safety concerns. If they were to put themselves in the auction today, it would very likely be a repeat of the 2010 auction where a total of zero Pakistani players were picked up despite even being shortlisted, including former purple cap holder Sohail Tanvir who was very critical of this move. In that case, what could even be legally done? IPL owners do not bid for several good T20 stars in other leagues too, the likes of CdG, Maxi, Lewis, Fizz have all gone unsold in recent times and you cannot pursue a legal case (in what court anyways?) accusing the owners of bias when they could simply point fingers elsewhere at the foreign quota limitation or the wage cap. I don't believe in the theory that Pakistani players won't have anything to offer to the IPL, someone like Shaheen Afridi, Wahab Riaz, Shadab Khan, Asif Ali and even Babar Azam are likely to be picked up if they could be snapped up cheaply. But, will any owner risk taking the potential controversial hit in the current political climate? Further, will any Pakistani player feel truly secure, both externally and internally to put themselves in the auction?
I also don't believe in this fairytale of Sport and Politics being two separate entities. No, through most human history sport has always been political. A football match in the Central Americas brought about a war, equally a football match in Africa laid the foundation for a civil war to end. Popular sporting events like the World Cup and Olympics have been utilized by host countries for 'sportswashing' their image to the global world (Nazi Germany, Argentina's military dictatorship, USSR's communist regime and the upcoming Qatar World Cup to name a few) and you've had a few athletes and countries even pullout or threaten to do so. If Sport and Politics were not to be mixed, you would have never had the apartheid ban on South Africa for two decades for one. Now do I wish for them to be so intertwined instead of being separate entities? No, I wish politics were not this influential on sport but to believe that politics and sports are separate entities which will not be mixed is an unfortunately naive stance IMO, just like how people globally are realizing that politics has always played a huge role in most facets of life ranging from entertainment to even science throughout human history.