All of the mefarshim are bothered by this, why was Tamar chayav שריפה? Rashi says that she was the daughter of שם and therefore a בת כהן (against the Gemara in סוטה), but in any case even so the din of שריפה should not apply.
The בעל הטורים quotes a fascinating pshat in the name of R' Yehuda Hachassid. He says that they were not going to kill her, rather they were going to burn a mark on her forehead to mark her as a prostitute.
