A lot of people don't know the distinctions. As a freelancer, I can't tell you how many times I've been asked to "edit" something only to find out what the customer really wanted was a proofreader. As a general rule, I don't take such assignments. I find it really annoying for someone to ask me to proofread a 500-word article. What they really want me to do is make sure all the commas and periods are placed correctly and they've spelled the words right. That's about a 10-minute job, or less, if they're half way decent at writing. And what am I going to charge? Not much.

On the other side of that coin, if the writer is lousy, I end up re-writing most of the article. In that case, I should just write the article for you and charge my usual premium writing fee, which most people who ask for proofreading services don't want to pay.

For these reasons, I usually have a minimum order that I expect so that I can ensure my time is valued.

I can see the issue with scientific papers. Most academic science writers aren't good at writing. They need proofreaders, but they can't afford to pay them. A proofreader for such documents would need to be a specialist in the academic discipline, which means they would charge more for their specialty. In my experience, academics don't want to pay for people to proofread their writing. I'm not even sure that most of them understand the value of a proofreader. I've seen some poorly written academic texts.