The issues are as follows:
Profiteering of someone else trademark is illegal.
PPC typo sqautting pages are actually an improvement on adult pages typo squatting the same domains.
The issue of what is and what isn't a typo is unclear.
A lot of what Sgt KickAxe says is actually very true - major corporations that allow their domain name to be typo squatted are also somewhat to blame for the typosquatting. My earlier examples took one of the biggest names on the net at the moment and showed just how they have done nothing about typo-squatting (they could probably have put together a good case against the vast majority of these typos)
However I don't see them as entirely responsible - ICANN must take some of the blame by not making the UDRP more accessible and easier for people to pursue. The parking companies must also clean up their acts (but when it comes down to it - I'd rather see their parking pages than adult content on these sites)
On the other hand I don't see it as being none of the business of ordinary webmasters - ignoring a problem and hoping it will go away is not a course of action that I would recommend to anyone - should you ignore drug dealers on your street corner or racists meeting in your town square. Ignoring parasites just allows them to breed and become more of a problem to their host.