I find that reasoning suspicious. Ever notice how most of the muni and coop internet providers offer a symmetrical speed while att and comcast don't? My bet is that there's no real technical limitation, it's simply price segmentation.
There are technical reasons for offering asymmetrical speeds when it comes to coax connections. There's only so many channels on the wire, a lot are still dedicated to television, people mostly only care about download, so they prioritize it. PONs don't carry television and are just fundamentally different in design, so its easier for them to support symmetrical bandwidth.