Well it shouldn't, because that is literally against the spec.
Foobar2000 has volume control, and - once again - straight from the spec:QuoteIf a player chooses to apply any volume adjustment or gainBy being able to have volume control and not apply the header output gain, foobar2000 is literally breaking the spec.
modification, (such as the R128_TRACK_GAIN (see Section 5.2),) the
adjustment MUST be applied in addition to this output gain in
order to achieve playback at the normalized volume.
I do think you are misunderstanding this. See my last comment: this specifies that the gain tags are being applied ON TOP OF the output gain, and that the need to combine these operations is a hard requirement to get the right result. Hence the MUST for "in addition to...in order to achieve playback at the normalized volume". It's important that the implementer understands these need to stack or they'll get the wrong outcome when switching Track Gain to Album Gain and the reverse.
Saying that a player that has a volume control is not spec compliant if it offers an option to ignore that tag is not just uninteresting, it's easily verified as wrong because the spec does use SHOULD and not MUST for applying the gain in the first place (see, I can RFC2119 language laywer too!). This very exact point is literally *spelled out* in the spec revision here: https://mailarchive.ietf.org/arch/msg/codec/zctRSJ84-Gd5G3XSUcNB-lAGSO0/