A number of good reasons have been named but it's also conceivable the EU does not care that much. For all we know, it could be prepared to substantially reduce this payment as part of an agreement on some other issues. Similarly, the Commission has been adamant the ECJ must play a role in any future dispute on the status of EU citizens in Britain, which is unheard of and does not seem plausible. Even EEA countries like Norway haven't agreed to that, instead relying on a [separate court](https://en.wikipedia.org/wiki/EFTA_Court). And those are things that nobody had been talking about before and that do not even touch to the core issue of the relationship between the UK and the single market.

The thing is that, as long as it remains united, the EU has a very strong negotiating position. The UK has a lot more to lose from a hard Brexit and the clock is ticking. So the EU can afford to make anything and everything an issue and might just be preparing ammunition for the next rounds of negotiations. It has no reason to concede ground before the UK has put forward specific proposals that could be negotiated down as part of a quid pro quo and EU diplomats have been complaining loudly that such specific proposals are still missing.