@holm This is not unusual at all: except for the Belvedere (the odd one out), you can take photos at any art museum in Vienna, for example.
Also in Paris, at the Musee d'Orsay you can take pictures to your hearts contend.
And the Rijksmuseum in Amsterdam even encourages it! They even post super-high-definition images on their website for you to download! Check it out RIJKSMUSEUM - and when you sign up (sign in) you can start your own on-line collection!