Designs of any sort can be copyrighted – it usually lasts for a period of 25 years so many of 1970s designs can actually be outright copies if the modern company is capable of it (which it usually isn't). I have been at the sharp end of this as a designer myself when I had to take a company to court for breaking copyright on a surface pattern design I did
Here is a link (it is to a company that explains the copyright law and is in no way meant to advertise that particular company)
I am less sure about the renewal of design copyright; I think it is in the link below in one of the long legal documents
http://www.jenkins-ip.com/patlaw/introdes.htmThis is of course UK design law; I forget it may differ in other countries
I agree that companies or individual glass makers can often be seen as being influenced rather than copying older glass - they still have to watch if it's within the 25 year limit though, I am sure modern Daum or Lalique would be quick to act on the closest similarity to one of there exclusive items
With retro glass on a high at the moment more companies in Poland and China are being tempted to create items with the very strong influence of Whitefriars, Riihimaen or Holmegaard. I think sometimes they copy outright if there is a market, the item in question could be a case in point