Santorum has a microscopically thin but nonzero chance of winning the Republican primaries. To do so, either Romney has to all but disappear as a candidate or he has to do extremely well in the upcoming primaries. Even then, the most likely outcome is a "brokered convention", where literally anything is possible, up to and including nominating someone who isn't even running yet.
Santorum knows that he has to win the GOP base this way. And he'll never win the libertarianish part of the GOP base to begin with (they're voting for Ron Paul), nor will he win over the sane people (they're either voting for Romney or staying home in a fog of crippling depression). So his only chance is to maximize turnout from crazy, evangelical, right-wing loonies of the type rarely seen outside flyover country.
If he somehow wins the nomination, expect him to quietly forget half of these crazy promises. If he loses, his political career is over. So he has to go pretty crazy.