I think all of those reasons are why they're pushing for venues to be considered the vendors, they know they can't enforce it in any real way, but if they at least get all of the major sites on board they'll get a decent chunk of change out of it. 

When the EU introduced VAT on digital goods sold by overseas sellers, I know there were at least a few third party sites that announced the changes and said it's not our responsibility, each individual seller has to collect and remit (eg Bandcamp, the other 'E' site). There was huge backlash from the sellers, until those sites said, ok then, we'll collect and remit (in other words, the majority of independent sellers don't much care if someone in another country has to pay tax, they just care if they have to do anything about it). There's a sales tax also applied now to every sale I make to a resident of Washington, so the sites have the ability to do it, and there's precedent.

With the EU VAT, a seller can register to use a simplified version and avoid having to work out each individual member's rates and what's owing on each sale, and the revenue just gets distributed after they hand it over. It might be years away, maybe even decades, but I wouldn't be surprised if eventually, there's some kind of global taxation system that works similarly and that catches nearly everything in participating countries, much like other global agreements like the UPU etc.