It's a weird problem actually. There are valid reasons for blacklisting and whitelisting free email providers.

Some sites only allow registrations from private domains. They blacklist all the free email providers, which makes sure that mostly businesses, academics, etc. are signing up for their services, rather than randos who may have little to no value as a user.

However, some sites see the randos as the only valuable users, and sometimes see private domains as a threat since a bad actor could use one to spawn an infinite number of valid email addresses for registering accounts. Free providers make it much harder to create a new address, so they whitelist them.

It's likely because they use it as the primary/unique identifier for the account, which is just dumb. It's like they've never heard of a UUID/GUID before.