Presumably the logic is:
1. The Le Touquet agreement allows the UK to put an immigration control on french soil.
2. According to UK law people can only apply for asylum once they are already on UK soil.
3. So asylum seekers wishing to get to the UK get stuck in Calais.
4. Somebody has to pick up the bill.
5. Currently that somebody is the French tax payer.
6. Since these people want to get to the UK and the UK is not letting them, it could be argued that the UK is at least partly responsible.
I should say, I don’t know whether there are already provisions in the Le Tourquet agreement to share costs.