How about this way to tell apart on-topic and off-topic SaaS questions:
Criteria: Is the web application software required to build such a SaaS service available off-the-shelf (commercially or freely) for on-premise installation?
Yes -> Hosting question, off-topic
No -> Webapp question, on-topic
Applied to the case of the SaaS machine learning question: Even though these machine learning tools exist for command-line, there is no ready-made webapp that I could install on my server and that would immediately (without custom development) provide machine learning service over HTTP. So: on-topic.
Applied to an hypothetic question "Is there a free wiki SaaS?": Anyone can install Mediawiki on their server within a day and start selling instances. So: off-topic, close and tell asker to look for Mediawiki hosting.