Question regarding how Amon took away people's bending.

What we know:

• He's a bloodbender
• Can bloodbend with his mind
• Takes people's bending away with bloodbending.

What im wondering is, why did he need to physically touch people and in that specific location with that specific part of his body? Afaik, no other bending technique requires you to physically touch the person, and waterbending is always done with a distance (even if you can't see the water, e.g. bloodbending). Was it just an act so people didn't question him? But then why would he do it to Korra when no one was around?

Very confused.