How does a hosting platform submit all of their sites to search engines?

Google's webmaster tools help says that platforms like Wordpress (and other widely used platforms) "probably already" submit your site to google for spidering.

Is this actually true? Is there an API or way I can have my home grown platform to submit these sites to google? I don't want my clients to have to sign up for webmaster tools on Google, Bing, etc, just to start being spidered.