That's a terrible comparison. Amazon is a retailer, Twitter is not. Amazon provides a massive amount of helpful information to buyers. Twitter provides almost none. For starters.

For people with significant followings, Twitter is a marketing platform. This might enable an affiliate (or self publication) model to work well on Twitter.

Amazon has an established reputation as a low-priced retailer, and stakes its reputation on that. Twitter seems to intend this as more of an ease-of-purchase kind of feature. My expectation is that there is little to no incentive for Twitter to be pricing these products competitively.

Not everyone comparison shops for all products. Many people are happy paying a standard price for certain goods, without hesitation. For example, expecting to pay $10-15 for a t-shirt is common, as is paying $1 for a song. For these sorts of purchases, reducing the friction from discovery to purchase is pretty important. Suffice to say, I probably wont buy a car or a vacuum cleaner on Twitter.

I buy things off Amazon because I go there to buy things. So yes, I guess I use that button. :)

Amazon does use more than 140 characters to present a product.

And often has reviews on the same page.

The reviews are the key. I always jump straight to the reviews. Half the time I never read the item description.

You can always add a link to the tweet for more details.

And then you can buy from the page with the details. Forcing you to go back to the twit to buy would be a bad idea.

Embedding the buy button wired up to your twitter account might not be so awful?