This is to comply with customs and excise laws.
Er, no it isn't. If the flight is in the EU the applicable VAT would have been paid in the EU country of purchase. There are no 'duty free' aircraft stores for intra EU flights.
I don't know of any country that have laws that prohibit using duty free stores after boarding. For example on a LHR-YVR flight, while we sat for four hours awaiting de-icing, BA served a meal, and a full drinks service in all cabins.
This is to comply with FR policy.
If they are telling you it's the governments fault, they are either uninformed or they are lying to you. It's ironic, if you are delayed before boarding you are entitled to 'care' meaning food and refreshments, etc. but this doesn't seem to apply once you've been seated.