Almost all companies default to either to a subdirectory or subdomain when desiring to offer a quick way to get to a page.
Subdirectory
Amazon.com/amazonprime
Subdomain
music.amazon.com
There are positives and negatives to both of the above options (SEO, customer recall, etc).
However here we have a, somewhat unique, 3rd option. Since Amazon itself manages .now and obv owns Amazon.now, it could launch Amazon.com/now as Amazon.now. I strongly doubt the team that launched the subdirectory option sat in a room and decided to consciously choose the subdirectory option. Instead, they simply defaulted to it - likely unaware Amazon owns and manages .now and thus in this regard had an alternate option.
In most cases, when Amazon (or any other company launch products) there is no 3rd option. They have to choose between a subdirectory or subdomain. This is true for .com and all other extensions, such as .org, .ai, .co, .net, .app, etc. They almost always must choose one of the above options. Only in cases where the name of the product service exactly matches AND a gTLD exists, AND where they own it (or can acquire it) - do they have this 3rd option. It's a rare case.
Over time, as Amazon's internal team becomes more aware of their own registry and options avail to it, reconsideration of the URL they are using may be forthcoming.
Personally I wouldn't read into this as a conscious decision one way or the other. Just a default decision, made without knowledge of what could be perceived to be a more optimal decision.
