How often do publishers sue researchers for copyright infringement for putting their articles on a personal website?
I have never heard of an individual scientist being sued for posting a few articles online (large-scale systematic activity is a different story).  So far, unlike the music and movie industries, scientific publishers appear to have simply not found it worth the cost in resources and community relations to do so (though they've been quite comfortable suing organizations on occasion).

If a scientist decided to resist a takedown order, however, it seems possible that a journal might feel forced to make an example of them.