Open source is known for having less bugs and vulnerabilities per line of code in comparison to proprietary software. This was one of the main reasons behind mass migrations away from internet explorer to firefox.
Open source is also known for being more innovative. Linux was the 1st out of the box operating system to come equipped standard with a firewall. Forcing microsoft windows to follow the trend.
If ISP's become liable for content posted by end users. It sets the stage for large numbers of private citizens being banned by ISP's for publishing certain types of content.