In Canada the taxpayers paid for the network up until about 25 years ago. After that it was privatized and every cent spent since has been done by the private companies, not the taxpayers. Given that the past 25 years has included huge amounts of broadband build outs (it effectively didn't exist before that) and entire new technologies (fibre to the home) this becomes a much more complicated question.
What incentive will a big player have to develop it's broadband infrastructure if they legally have to give subsidized access to it to their competition? That's actually the state of affairs right now in the copper/ADSL world. The prices that companies must offer to their competition to use that network don't cover the cost of building or maintaining it. This has been justified by the fact that the original copper plant was built with taxpayer money, however even that is a bit questionable because a lot has happened with private money since (including all the ADSL equipment, and in many cases the wires themselves (any community less than about 25 years old, or anywhere that required replacing the wires for any reason). Fibre to the home platforms have been, up until now, exempt from these sharing agreements, a competitor is welcome to try to negotiate directly with the incumbent for access, but there is no legal requirement that the incumbent allow it. These fibre networks are all new enough that they were paid for 100% with private money, not taxpayer funded. The current CRTC hearings are discussing whether this should continue, or if the fibre too should be required to be open to the competitors at discount prices.
Unsurprisingly, the incumbents don't think they should have to share the networks that they paid for with their competitors, and in any other field it would be considered ridiculous to even ask them to. But there's also the obvious question of how many lines we want to run to each house? We don't want 100 competitors all running their own wires.
I think the best outcome for all involved would be if the incumbents do share their network with their competitors, however, the mandated prices must be more than the cost of building and maintaining that network (which is not currently the case on the copper side of things) I also think that if they do this, it should be done evenly. Currently only telephone companies must share their outside plant, cable companies are immune, as there is no difference anymore between the products provided by either (phone/cable/internet) or the technology used (fibre optics) there is no reason to give the cable companies preferential treatment.