When Advanced Threat Protection for EOP was released it could be purchased as an add-on service via the Office 365 admin portal. I added it to my own E3 tenant at one stage, before I later upgraded to E5.
I'm currently working with a small business customer who could use the extra protection of ATP, particularly Safe Attachments. They are running Office 365 Business Premium. Neither they, nor any of my own prod or demo tenants, are able to purchase ATP as a standalone add-on today.
I guess I found the reason, which doesn't sit well with me:
"ATP is included in the E5 subscription. If you don’t have an E5 subscription, in order to begin using the safeguards provided by ATP technology along with your Exchange Online service, you need to purchase a separate subscription for ATP. You can order ATP through the Microsoft Online Subscription Program."
What possible justification is there for limiting the customer's choice like this? It can't be to make things simpler - the "Purchase Services" page still lists 52 other items they can buy, some of which cost less than ATP did.
So they need to either increase their costs by more than 2x to get ATP, or sign up to a VL agreement they don't want or need. Even if I can get them to accept the cost of E5 licenses by making use of other things that E5 includes, they don't want to move off Business Premium because they're excited about the new Microsoft Bookings app that is not available for E* tenants.
Surely making it easier for customers to buy valuable services like ATP should be the priority here.