In the last few years, I have seen some sites such as directories, blogs, top lists, tgp's, etc. that build their traffic on recip links and bookmarks suddenly change the site into a paysite....and of course capitalizing on all those inbound links, bookmarks, brand, built under the previous incarnation of the site..

And, it appears that the switch to a pay site model is usually done without notification to those inbound links that they are switching their site over to this new format..

Is this a strategy that is ethical to pursue?

Is it good business practice?