So, Instagram is the only social media I use (generally to share photos from our many road trips). My account is private, and I only follow people that I know in real life. I suspect this change won't affect my experience at all, because I'm not consuming much of anything aside from my friends' photos. I suspect, however, that I might be in the minority as far as Instagram user habits go...
This opens the printer up to potentially massive copyright infringement issues. Sharing images online is one thing, being directly paid to actualise then on offline products is a big step with a lot of potential liability.
There's already lots of competition in that space, the sites I've used for Tshirt & mug printing (in Europe) have been very hot on trademark and copyright issues, stiflingly so IMO.
There's probably much more to be made selling pictured goods over good with pictures on.
For instance, I follow King Arthur Flour for their scrumptious baking recipes. It'd be great to be able to go straight to their online store to purchase some of their harder to find ingredients like Boiled Cider.
One thing I hope they do is make it so everybody can get in on it. Say you want to be able to sell prints of your great photos, how awesome would it be to have a built in way for customers to do so? Or have a brewery and want to sell some t-shirts?
That said... will this taint the instagram experience? Youtube feels increasingly optimized for profit-making and seems be crowding out some of the more "just-for-fun" type projects. If too much of a social media site focus on profit it becomes just another mall and less of a social setting.