-Probably placed at the upper end of the passband so you could use a single high-pass filter in front of the detector to get rid of any noise on the line, easing detection.

Also, if you wanted to remove the tones from the passband for some reason or other, you could do that with a single notch filter if the tones were sufficiently close.

Based on some quick Googling, it looks like the pass band they used for voice (at least for Apollo) was 300Hz to 2.3kHz. Assuming that filtering was being applied in the audio gear and the RF side of things was a bit more permissive, that would allow them to keep the control tones outside the range of any vox transmission.

so they could be removed with a single notch filter