It's actually not MS's fault this time. They had to make the change because someone patented the "autostart" of embedded content, and they didn't feel like paying the licensing fees.

The patent holders just haven't gone after the other browsers yet.

Read the linked article.

Basically, it's another (likely) unjustified software patent being used to hold companies (and users) up to extortion.

It's been known about for a couple of years. Web site designers are going to have to rework a lot of pages. Many haven't.