It's sloppy speech. With more time to think, or in writing, the speaker probably would have said:
Capital punishment is usually reserved for murder.
The mistaken wording probably arose because the speaker was trying to refer to the way that capital crimes are defined, but it's hard to think of a way to say this without clumsy grammar. By the time he's said "capital crimes" as the subject of the sentence, he's in trouble:
"Capital crimes are usually" (thinking…) defined so that…only murder can be a capital crime. (Oh, no, that would sound stupid. What's another way to say this?) The definitions of capital crimes are usually very strict: that category is normally reserved for serious crimes like murder. (But I already said "Capital crimes are usually". How do I get out of this? Uh…) "reserved for murder…and not just murder" (I need something stronger, to illustrate my point that we don't define just any small crime as a capital crime) "but murder plus some additional facts that make it particularly egregious." (There, that was technically wrong but I blabbered enough dignified-sounding words after my mistake that people probably won't notice.)
The relevant sense of reserved is the second one you listed, which is a metaphorical extension of the first sense. The idea is that capital punishment is the "last resort" in criminal penalties. Metaphorically, we have many criminal penalties, varying in severity from community service to paying a fine to a short prison sentence to a long prison sentence to death. We "save" death as a punishment for only the worst crimes, as if the death penalty were like expensive silverware that we use only on special occasions.
