Uh, because with a drawn symbol you wouldn't need their thirty dollar notebooks?

You'd still need the pages with the "special dotted paper pattern".

Does evernote not allow you to fix the page manually? (I don't use it.) The JotNot Pro app I bought for a dollar allows one to slide the corners around and reskews the image as appropriate, with some amount of auto detection as well.

No, stickers could serve that function. Google "registration marks"

Not that it's impossible, but it would be significantly more difficult to make the digital signal processing algorithms to detect custom symbols, especially hand drawn custom symbols.