copywrite : Sounds like it should be the verb for the work done by a copywriter; to write "copy" for advertisements, book jackets, product boxes, and so on.
copyright : A word for the rights granted under the law to a creator, regarding control over what people can do with the creator's work and copies of that work.
1. "Copyright" is a noun, not a verb. You do not "copyright" something, you have copyright or hold the copyright for/on something. "Copywrite" looks like a verb, doesn't it? Using that misspelling spreads the common misconception that people need to take some specific action "to copyright" their work. That's false: copyright is granted to you when you have created something creative to which the copyright laws apply. It's not something you do, it's something you have.
2. "Copywrite" also sounds like it's about "writing", specifically; copyright is actually about a broad class of creative work including writing, drawing, music, software, etc. Using the misspelling "copywrite" spreads the common misconception that copyright is meant just/mainly for written work.
3. "copyright" does also have a verb use, which was much more relevant before the Berne convention made it unnecessary "to secure copyright for" one's work in most countries the recognize and enforce copyright. You're better off thinking of it as a noun, which it primarily is.
P.S. somechicksings contributes:
copyrite: A secretive ritual held under the full neon moon at Kinko's.