system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do negative prescription glasses require a prescription from a docter and cost hundreds of dollars where positive lens glasses can be bought for $3 with no prescription.
gpt
This isn't exactly true. What you have to look at is what the "negative" and "positive" mean. However, there is a distinction between that and prescription glasses versus non-prescription. 

The negative and positive reflect what your eyes need--negative is for people who are near-sighted (can see close, but not far), and positive is for far-sighted (can see far, but not close). The $3 glasses you see over the counter are reading glasses, which basically help people see those things close to them, such as reading material. They are all very weak and generic, not specifically made for a particular person. You can not do this for people who are nearsighted. If you wanted to, you could say that a nearsighted person could buy binoculars over the counter, but they don't actually help that much.  

Any person with a more severe sight impairment would need glasses specially tailored to their needs to see properly, and those cost much more money.