Percolation on Cayley graphs of groups is a particular case of percolation on transitive graphs.  Percolation on transitive graphs satisfies many important properties which do not hold in general (see 
<a href="http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CFIQFjAB&url=http%3A%2F%2Fresearch.microsoft.com%2Fen-us%2Fum%2Fpeople%2Fschramm%2Fpyondrep%2Fpyond-rep.ps.gz&ei=OuvVT8v-B4rF0QXNktn7Aw&usg=AFQjCNFKs9Rcw6_JxdH8jPbNMXRPB_OnXQ">this paper. </a> That provides a motivation. The Cayley graphs of semigroups are typically not transitive. Nevertheless some Cayley graphs of semigroups have been considered. For example percolation on the NE quadrant of the square lattice (which is the Cayley graph of the free commutative semigroup) is considered <a href="http://arxiv.org/abs/1205.5873"> here. </a>