It's a simple answer: Widowers are free to remarry.
I seem to recall a specific prohibition in Church law if the wirdower killed his wife. He may not be validlly married.
Schiavo was never officially accused of murder. It seems very likely that he did murder his wife, but the Church has to go by what the justice system says about these things, and Schiavo was never investigated.
Presumably he never will be investigated, since the killer judge in the case allowed the body to be cremated after a coroner approved by the Scientologists had a look at her.
The only real impediment to a church marriage would be if the woman did not get an annulment after her divorce. Presumably she did. Either that or the pastor and bishop never bothered to investigate the matter.
In the strictest sense, it is true that a widower can remarry, and it is also true that Schiavo didn't actually murder his wife in the legal sense of the word. However, I believe it is also a priest's prerogative not to perform the wedding. But there are enough leftist Culture of Death priests out there who are willing to accomodate this parasite, the priest and Schiavo should be excommunicated.