From a service provider's point of view [1], a big portion of "users" with disposable email domains have intention to do bad things, e.g., spamming, aggressively scraping contents... It's not easy to distinguish users-who-want-to-try-out-then-never come-back from users-who-want-to-do-bad-things-to-your-service.
Typically, online service provider may have empathy towards users with whatever emails, including disposable emails for privacy reasons. Then the online service becomes popular, and spams / scraping activities become out of control - Internet is big and there are a lot of bad people / bots. Try different tactics to fight bad users / bots. Eventually, the service provider joins the dark side and bans the use of all disposable emails. It's not ideal. If you are Google or Amazon, maybe you can trivially allocate 20 full time engineers to develop an elegant solution. For small businesses, you just use very limited resources and do whatever to survive.
[1] I run listennotes.com