re: St. Louis couple who defend their home have guns taken

If this is the weapon(s) involved in the case then it's not at all unusual for that to be seized as evidence. Several things can happen from here:

If case dropped or found not guilty at trial, that evidence will be released and returned.

Or there could be a deal wherein case dropped in exchange for voluntary forfeiture of only the weapon(s) involved in the case.

Then of course there's the possibility that if found guilty pursuant to a trial, weapon(s) would obviously be involuntarily forfeited at that point.