Why a Subscription?
It's Simple. Costs.
There are both fixed and variable costs to developing and hosting the ShelfScan web app, and to providing an iOS extension on the App Store. I am currently unable to pay these costs out of pocket and I believe that the value you can get out of ShelfScan is much higher than the price of the subscription.
If you can benefit from the ShelfScan application and are a non-profit volunteer, open source contributor, or similar, or can (actively) assist in the testing of ShelfScan, I am happy to discuss providing a free subscription.
Download the Extension for Firefox on Android