AFAIK, the French do not police the border in Calais
They do - exit controls, albeit sometimes cursory, are standard in France when leaving the Schenghen zone.
Despite what the British papers would have you believe, much of the cost of security at the ports and at Coquelles has been born by the French (both public and private sector). This is one of the reasons why ending juxtaposed controls is hugely popular in the Nord-Pas de Calais area, and an easy win for national politicians.
EmilyAlice I don't ever use Calais because if we use a short ferry crossing then it's always Dunkirk where my best friend lives, but yes - controls always fairly cursory before this year, now it's become ridiculous. The car in front of us - normal family car with UK plates - took nearly 15 mins to get through UK border controls (having passed through check-in and then French border controls normally) and they spent about 5 minutes minutely examining our passports - british couple with a kid travelling on French plates. It's now more hassle than flying. My friends in Dunkirk don't bother with shopping daytrips to the UK any more, as it takes so long to negotiate security that it's hardly worth it.