There's overselling and then there's assuming that the average user is only going to use 3% of their allowance! That seems pretty optimistic. Having said that presumably the wholesale rate varies quite a bit between countries, and I assume some contracts are offered as loss leaders.

One of my relatives in Italy is using Kena. It's solid. Even though they advertise 30 Mbps, we clocked it at 68 once. Generally it's more than enough to watch Youtube, which is where most bandwidth is spent, unless you hotspot to a laptop.

The point about the overselling / oversubscribing for consumer rates actually goes counter to your point since every customer (consumer or business) all have the same priority in European networks.