For what reasons would you not include a volume control in an audio player?

Both Bandcamp and Soundcloud, two popular audio streaming services, don't have very accessible means of controlling volume. Bandcamp doesn't have a control at all, while Soundcloud only provides one on their website; the embedded widget doesn't have one.

This decision causes a bit of annoyance for me, because to change the volume I need to either adjust my system volume or my external amplifier. This throws off the volume balance on every other audio application I use.

What's the reason behind not including a volume control? I know I'm just one user, but I don't think my problem is particularly obscure and I don't see any upsides for not including one. I wasn't able to find an answer from Bandcamp and Soundcloud, and I wasn't able to find any research on the subject.