What are their goals? I'm assuming just to raise revenue?
Is it even a clear problem to figure out which business pays what?
The ISP is responsible for collecting the tax so they would know the sources.
Only from france-based uploaders , right? This applies also to external ones.
Let's say that traffic (anything: ssh, sftp, http) enters a VPS in OVH in France from a laptop in Italy (Italian ISP) and some bot in the USA (eg googlebot) or some real person download something from that VPS (US ISP). Who's paying what? There are sent and received bytes for all parties.