Withdrawal of the UK and EU rules on .eu domain names [pdf]

If your country elects to leave the EU or possibly just gets into a spat with it, you will have to abandon your domain name and thus all accumulated pagerank, external links, email addresses on business cards etc. Although the EU could easily just block registration of new domains to non-EU homed businesses, they are choosing to go much further and take away existing domain names that are live.

This is entirely consistent with the EU's behaviour in other areas - use whatever bureaucratic mechanisms it has at its disposal to cause as much annoyance or chaos as possible to any country that decides to leave, in the hope of dissuading the others.