[Archelaos]

> why not include the possibility of a national veto at implementation stage

In case of such a veto of a single member, the whole legislative process would in the general case be set back to were it started, because the other members had agreed under the condition that everyone implements the agreement and other countries might want to renegotiate.

The EU's ability to force a country to a certain legislation goes even further: In the early years of the EU every country had to consent to an EU legislation; today, under the Lisbon Treaty[1], EU countries must implement an EU regulation in certain areas even if they voted against it. This is based on the belief that a union of 27 states would otherwise hardly be able to act.

There had been cases were single members had been granted specific exceptions (most notably the UK, during its membership, and Denmark when it joined) or must meet certain conditions before they need to implement something (for example, in principle every EU country except Denmark must introduce the Euro when it meets certain criteria; but the EU tolerates that Sweden deliberately fails one criterium, since it has voted in a referendum to stay out of the European Exchange Rate Mechanism II). But these exceptions had been agreed on beforehand and some date from a time when the sovereignty of the individual states was valued more highly. Today, the EU tends to avoid such special rules for general cases as far as possible, because legal fragmentation is seen as problematic.

[1] See https://en.wikipedia.org/wiki/Treaty_of_Lisbon