Execute detects Spectators

I was flying while in Spectator mode over an area which was being detected by a command block with the execute command and the command block detected me. I don't know if this is a bug or a feature, but for me it looks like it's not intended this way.

What I expected to happen:
Nothing.

What really happened:
I got detected.

Steps to reproduce:
1. Set up a command block with a working execute command. (My command: /execute @a[-158,83,237,r=3] ~ ~ ~ setblock -162 76 240 redstone_block 0 keep)
2. Go into gamemode 3/Spectator mode.
3. Fly over the area that the command block is detecting.
4. Get found by the command block.