It says in the article 'Lightly trafficked roads or locations that just don’t make socio-economic sense can be excluded from the requirement' for the comprehensive network.

It'll be up to each country how they decide to implement it. In some countries the core roads are managed by a central government agency so they would need to arrange it. In others it is the the local municipalities or privatised. Some countries will offer to private companies. Others will provide it themselves. The governments would be ultimately responsible but Im sure they can manage this given they already have responsibilities about maintaining the roads and rest areas. In the worst case they might have to pay for some infrastructure themselves but can make it back with the charges.