Apple can charge 30% for apps and content because the ENTIRE product is being delivered on iOS. They can't charge that cut for SaaS because only a subset of the product is being delivered.
Could Apple attempt to take a cut from SaaS products if a user signed up from within your app IN THE FUTURE...sure, but I doubt they'd be stupid enough to try for 30%. Should Apple be compensated in some way for hosting/downloading your mobile app for your SaaS backend...maybe, and I'd be prepared to pay some commission if it drove signups, it might just be a tad lower than 30% though.