The blocking / throttling is the most important part because of the lack of competition. A server can burn through available ISPs if they don't like your site for whatever reasons. If a client's ISP doesn't like your site they can block it and prevent them from connecting. There are many clients who may only have 1 or 2 options for an ISP and switching over a single blocked site is not a big enough reason to switch. In cases like this I just use Tor and hope the exit node has an ISP that doesn't block the site.

Allowing ISPs to all have their own standards on who they block can be frustrating for both end users and site operators.