Basicaly if you are not reselling or distributing the fonts themselves, you are fine.
Fonts are treated as software. So the copyrighted thing is the font file. Not the font itself. You can't be sued for using some font, for creating poster (if you bought the font, seperately or with OS). You can be sued for sending the font file to someone else.
The people are buying Helvetica, because there are many versions of it. Reinterpretations (for example small text type is digitaliased from the prints, so you cant see the details that much, so Garamomd digitalised by three seperate type designers will look mean three versions) versions with more glyphs, weights or even proportions.
Also you might have to buy license for your client that wants to use Helvetica as his bussiness font. When he is using it to write in Word, each computer needs a license (when you buy fonts most have licenses for up to 5 computers).
When embedding fonts in software, mobile apps or websites using @font-face (dont mix this with using universal system fonts like Arial or Verdana, those you don't have to embed because everyone has them on their computer) you need special license. For this you mostly have to contact author of the typeface.