The UK Border Agency have a checkpoint in Calais, France; is this checkpoint UK territory or is it French? Whose laws are enforceable within it e.g. there is a notice saying no photographs, disobeying this would probably result in a charge under the UK prevention of terrorism legislation, but is this valid in Calais?
You can pass through France, Belgium, the Netherlands into Germany with only EU markers to show the borders, it is only the UK where the passport is checked.
The Calais customs and border area is UK territory, much in the same way as an embassy in a foreign city is designated as land of that embassy's country. I imagine it would therefore be subject to the UK's laws and restrictions.
The reason why different procedures apply at the UK border is that the UK is not a full signatory to the Schengen Agreement which effectively abolished border controls within the EU and a few other countries.
Of the 27 EU nations only the UK and Ireland are not signatories, although Bulgaria, Cyprus and Romania are yet to implement the arrangements. As well as the 25 EU states, Iceland, Norway and Switzerland are also signatories to the agreement and have implemented its rules.