Because technically, it's a scanner + printer and not a mover. It's just that the original is deliberately destroyed every time (unless something goes wrong *looks at Miles*).
IIRC, it's even mentioned that people going through the transporter are cached. Like, they store snapshots, so you can compare the changes between older you and newer you. You can also store a person in buffer digitally, meaning you can copy and print as many as you want.

I mean, just because it happens fast or in an unnoticeable way doesn't mean it doesn't happen. If somebody vaporized themselves instantly it's still suicide. Sure, people would probably accept the fact that their original is being killed for convenience, but the fact stays.