Porn was just an example, there could be any number of other sites that host content that an advertiser may not be willing to have their product/service promoted on, while the site may still be within Twitch’s guidelines for Embed usage.
That was also one guess at why Embeds don’t run ads. Either way, the PSoD seems to be here to stay so if you want to watch off-Twitch (or on-Twitch with some poorly designed ad blockers) then you have to expect it from time to time.