We’re starting to roll out the ability to add anyone as a guest in Microsoft Teams. This means that anyone with a business or consumer email account, such as Outlook.com, Gmail.com or others, can par...
Updated Feb 28, 2018
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You own your tenant's data. You control that data, so you have the chance to say if you want users from other tenants to access data in your tenant. It's your decision about whom you collaborate with (and Office 365 supports other features like collaboration white and black lists to allow or block specific domains). You do control whom you collaborate with in the context of your tenant.
But when it comes to other tenants, they control their data. And the mechanism used to allow people (guests) from other tenants are guest accounts created using Azure B2B collaboration. I don't work for Microsoft and don't work for Microsoft so I can't represent their views. If you want things to change, come up with some cogent and well-argued reasons why another approach is better (and more secure) and post them on UserVoice. You might get more support there than you're getting here.