Why has Macron/France insisted that the UK must nominate a commissioner?

Question

According to some Elysee officials quoted by the Guardian

All weekend, France took the initiative with Germany, Ireland, Donald Tusk’s team and a few other countries, to fix the terms of the extension very precisely: that the withdrawal agreement isn’t renegotiable, that the UK would follow a code of conduct and allow the EU’s 27 members to meet to discuss other issues for their future [such as the budget], and that the UK must legally appoint a commissioner if the European commission sits before the UK leaves.

Why has France insisted on this latter issue, i.e. the commissioner nomination?

Previous news on this matter suggested that this wasn't much of an issue in practice:

Sir Tim Barrow, head of the UK’s delegation to the EU, said in a letter to the Commission and Council that his government will not put forward a candidate’s name by today’s informal 26 August deadline.

In his Friday letter, Sir Tim cites Prime Minister Boris Johnson’s 25 July pledge not to pick someone to serve in Ursula von der Leyen’s incoming administration, which is set to take office on 1 November.

[...]

But what remains less clear is what would happen if the UK requests and is granted an extension to the 31 October deadline, given that the new Commission is likely to be largely on the starting grid by the time the next Council summit is held on 17 October.

Current UK representative Sir Julian King, a politically unaffiliated diplomat tasked with the security union portfolio, could yet be reappointed to be a place-holder during an extension period, although the tone of Friday’s letter makes it less likely the decision can be rolled back.

So I'm struggling to understand the emphasis of the French request. Is the EU totally unable to reappoint King without an explicit UK request?

Answer 1

Member nations of the EU are required to appoint a commissioner. The new EU commission starts work on 1 November. The UK did not nominate a new commissioner because it was scheduled to leave the EU on 31 October. Now it is not leaving until at least 3 months later, a commissioner is required.

Answer 2

It seems it's to make things as kosher as possible:

Von der Leyen’s view is that the UK would need to appoint a commissioner if Brexit negotiations were extended past the 31 October. Article 17 (5) TEU of the Lisbon Treaty removed that obligation, saying that any new Commission must have members from at least two thirds of EU countries. But, before the treaty came into force, member states had already decided to revert to the old process of appointment and insisted that any Commission must have a member from every EU country. This was turned into a legally binding decision in 2013, meaning that the UK would be required to appoint a commissioner if it were still a member state past November. But the penalty for non-compliance would be an infringement procedure against the UK, not expulsion from the EU.

EU and UK leaders could decide to make a derogation for the UK under Article 17 (5) TEU and allow the UK to continue being a member state without having to appoint a commissioner. But this would require the consent of all member states – including the UK. Likewise, the EU could refuse a British request not to appoint a British commissioner. But there are reasons to believe it would make an exception – not least to prevent complications in the process of approving the new Commission in the autumn.

Or in other words, because the EU would rather not deal with the alternative(s).

Also of note, before the EU struck a deal with Johnson and there was talk of the UK "sabotaging" the commission:

Jean-Claude Piris, formerly the director-general of the EU Council’s legal service, said the idea – refusing to appoint a new commissioner – would fail to shut down the EU, as No 10 hoped.

"The Commission can continue to work and decide legally,” Mr Piris wrote on Twitter, citing a precedent [Bangemann] dating back to 1999.