If fedex ground drivers were allowed to carry packages for Amazon on the same vehicle, it would be a great deal for them. But needing to have two vehicles crisscrossing routes all day simply means double the costs.
I see how it would benefit Amazon and Fedex to have contractors work for both, but it may also stabilize the ISP with the ability to swap drivers around based on need. Depending on the payscale, it could also mean that both companies would have more leverage over the ISP, because the ISP would have twice as much invested.
But I would think that there are local courier companies who would compete with ISP to service Amazon, so it won't simply be fedex contractors looking at this. And the fact that you are required to lease a van from them will make the case for real independent contractor status harder to make. This would look more like a franchise than even fedex ground, especially with the 'buy-in' arrangement. If someone 'buys in' will they have protection in the future via a guarantee of keeping the contract absent failing to service, or will Amazon reserve some right to cancel your contract and give it to a larger contractor. If I had contracted to service my home area, but a large contractor ended up with all the surrounding area, what protections are offered. Not everyone wants to have the hassle oh operating 40 vehicles.