Actually, they don't want to pollute the internet routing table with routes that are fully subsumed into other routes. The effect on address ownership is a side effect.

Actually, they just want to milk the money out of you. It's a matter of how much your willing to pay, as a business customer, it's all possible.

Most ISP do not have such pure goals, as to protect the global routing tables ;)

RIRs, not ISPs, allocate addresses at the top level, they make money on each address allocation, and they still won't allocate addresses to you if you don't multihome because they have a duty to conserve resources.

When you get PI addresses your LIR/ISP just passes your data on to the RIR.

Just like many industries there's a retail side and a wholesale side. You're asking to get a wholesale product from a retail channel. If you become a wholesale customer you can get what you want, for a price.