Because magnetic fields interact with electrically charged, or electrically conductive things too. Strong enough magnetic fields will induce eddy currents in your brain and make you taste metal (don't have a reference handy; this is a known symptom of certain high-field MRI's and there's probably an HN thread about it). Turn up the field a few orders of magnitude, and the Ohmic heating from induced currents will vaporize you.
Magnetars are so far beyond merely "vaporized", there's no adequate language for "deforms and tears apart the electron orbitals of your constituent atoms"; I guess "melt" is close enough for pop science! Basically, you're no longer "matter" in the conventional sense of the word -- you're no longer made of (recognizable) atoms.
Spaghettification is about gravitational tidal forces pulling all your molecules away from each other.
Magnetar vaporization is about the electromagnetic force (which is much stronger than gravity) tearing your molecules themselves apart. (Although the gravitational tidal forces around a magnetar would also be unpleasant.)
Magnetar vaporization would not be painful: Pain requires nerves, and your nerves would be gone. It also requires a brain to receive the nerve impulses and that would be gone too.
Spaghettification could be painful especially around a medium-sized black hole because it would happen slowly enough that you could feel your body being torn apart. Around a small black hole it would happen so fast that it would be essentially painless. Around a super-large black hole like the one at the center of our galaxy the tidal forces might be so gentle you'd never be spaghettified at all.
Magnetars are so far beyond merely "vaporized", there's no adequate language for "deforms and tears apart the electron orbitals of your constituent atoms"; I guess "melt" is close enough for pop science! Basically, you're no longer "matter" in the conventional sense of the word -- you're no longer made of (recognizable) atoms.