Plusnet doesn't want uncapped upload to save their bandwidth!
Not sure where you've got that from but it's not the reason at all. Speculating like that is where incorrect rumours start.
We were planning on doing it however one of the tools we needed to sort the process internally was broken and didn't get fixed until recently and other priorities have hit our workstack. It's still something we want to do but I'm not going to set a timescale to it.