I thought clause 11.4.5.1 in ACI 318-08 covered this. 'Perpendicular to the axis', i.e. legs across the width of the member? If its was spacing along the member it would say parallel to the member. But maybe I'm reading it wrong as we have a similar worded clause in the NZ concrete code that makes the distinction between 'along' and 'across'. I could have sworn in later revisions of ACI 318 that they further clarified it, but I cannot find my PDF to compare.
We have a similar clause in New Zealand concrete code as mentioned, but the spacing across the section for each leg need not be less than 250mm between vertical legs, unless the 0.33 x sqrt(f'c)b_w x d limit applies, then the lower limit is reduced to 200mm. In NZS3101 its really clear in that there are separate clauses for the spacing of stirrups along the axis of the beam and the spacing of individual legs perpendicular to the member axis. Our code is pretty much based on ACI 318 with some more stringent requirements due to the seismic side of things, so I'm surprised ACI doesn't cover it.
The reasoning for having relatively evenly spaced legs is so it prevents tension across the bottom of the section due to the strut angle for shear forces getting to the stirrups being too shallow. If you imagine a wide shallow beam with a single perimeter stirrup, there is a tension across the bottom of the section due to the shear having to strut at a shallow angle to the stirrup/corner bar.
See attached clause from NZS3101.