It's asymmetric, providing far less upload capacity than download.

It doesn't have to be. I'm on a PON, I've got symmetrical upload speeds.

It can be, but the two biggest PON operators in the US (AT&T and Verizon) offer symmetric speeds.

AT&T fiber in San Jose was GPON which is a 2.5 G down/1.25G up, split to several households (where I was, it was sixish lots per pole, but some lots had several units, and I'm not sure if the fiber to the pole was split earlier). They sell 1g/1g service on that, and it's clearly oversubscribed heavier on upload than download. 2:1 down:up is way better than most DSL ratios, and I usually saw better up than down speeds.