… which makes it the prospective subscriber’s problem, pushing nearly all risk onto them, and requiring them to trust that the service won’t spam them or sell their e-mail address.

What alternative do you suggest?

Depends hugely on the SAAS and it's current and desired customer base.

But some obvious candidates are

1) discontinue free trials.

2) provide enough obvious value to convert the current funnel of free trials into paying customers at a high enough rate that you don't care about the "freeloaders"

3) radically differentiate the support available on free trial accounts.

In the long run, and dev effort/time spend integrating email domain blacklists is just time taken away from building features that add value to the service/company. It's only possible that spending that time adding features will turn the conversion rate up, but its guaranteed that fencing off the top of the funnel will reduce the number of conversions.

A comparison of the dev time to check a value in an array and modify your entire business model is absurd.

Mitigation against abusers of a service is a valid strategy.

Emails don’t guarantee unique users. I’ve seen free tier signups that require a valid credit card or sms verification codes. It depends on the value of the free offering on whether or not putting signup barriers in place is worth it.

How do you know people use those emails to bypass payment?

I guess:

- multiple sign ups using different emails and similar name

- same ip address

- same data

etc.

Don't underestimate greed or laziness.

I have no idea how they know that. Perhaps they found a reddit or Twitter thread describing how to abuse their platform with anonymous email addresses.

Businesses that don't accept my email address don't get my business.

That's great, but there are very little businesses of 1.