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Q: I saw that [HubSpot does] not allow [AMP] to be used for landing pages or other pages outside of the blog. Is this because AMP is meant to be used for News/Blog style content only?
Short A: Pretty much.
Longer A:
>>Is this because AMP is meant to be used for News/Blog style content only?
If you read HubSpot’s How to use Accelerated Mobile Pages (AMP) in HubSpot and the cited How AMP Works page from ampproject.org, you’ll see they forgot to place a sign there. 
(see image)
While beautifully written HubSpot landing pages with no Javascript files, no stylesheets, no HubSpot forms, no head HTML and no footer HTML surely exist, can’t honestly say we’ve ever seen one in the wild or would ever really want to.
>> Is this something that will ever be allowed?
“Only the Shadow knows what evil lurks in the hearts of men.” haha
That being said, if AMP is ever allowed on other pages it will certainly be announced either on the HubSpot Product Updates Page, at Inbound, or both!