I'm German so I may be biased, but I don't think this is punitive at all.

The UK has been using the threat of leaving as a trump card in negotiations for decades. They've basically opted out of all major agreements yet demanded having a say in every decision. They've basically been given special treatment like no other EU member and still complained about the "fascist EU" taking away their "sovereignty".

UKIP was the logical consequence to decades of telling your own people everything that goes wrong in the UK is the fault of those evil bureaucrats in the EU. Cameron tried to bluff in order to renegotiate the UK's special privileges but he made the mistake of promising a referendum he didn't foresee he would lose. He severely underestimated how much those decades of FUD had paved the road for the Leave campaign.

In the wake of the referendum the entire UK government has committed to following through. The result has been communicated as "a democratic decision", not as a tentative opinion poll to aide in decision making. Nobody has invoked Article 50 yet but everything in how the UK has handled this says "we're going to leave".

What the UK apparently didn't expect is that the EU's reaction can be summarised as "Good riddance; don't let the door hit you on the way out".

The UK has two options now:

1. Follow through and renegotiate everything.

2. Admit they were bluffing and try to handle the fallout.

Either way the brexit trump card is no longer on the table. Staying in the EU would mean effectively giving up on all the advantages gained in previous negotiations because nobody is going to take future threats serious anymore. Leaving means renegotiating everything from scratch.

The UK isn't being punished. It's just having the privileges revoked that it previously bargained for by continuously threatening to leave.

There's nothing vindictive about this. There's just no reason for the EU to be nice to the UK anymore. How the UK is being treated now is simply a reflection of how the UK has treated the EU until now. It's certainly not nice, but it's not unfair. There's simply no reason the EU should negotiate anything in advance to Article 50 being invoked.

BTW, in Germany the general appearance of the Brexit seems to be that the UK made a huge mistake and their country is disintegrating as a result. Scottish independence is perceived as a given and nobody sees any long-term point of having EU companies stay in the UK unless the UK can reach an agreement (e.g. staying in the EEA, which would mean upholding a ton of regulations the Leavers complained about with no way of affecting them). Nobody is worried about this setting any favourable precedent in the slightest.

And for the record: despite Leave campaigners' propaganda, Germany has its own nationalist anti-EU movement which had previously gained a considerable following as a result of the refugee crisis, which has hit Germany much more than the UK). Germans don't see the EU as some Fourth Reich tool of controlling Europe. Most Germans don't even seem to be aware of the amount of political power Germany has within Europe. During the Greek financial crisis the general reaction to the prospect of a Grexit was actually positive (again with some worries about how badly Greek would do on its own). The mindset is simply different: Germans seem to genuinely think of themselves as European whereas Brits seem to think of Europe as "the continent" (i.e. not them).