I think it's quite clear Goldie, if you are an illegal immigrant who is a 'flight risk' , in other words you could try and hide from the authorities who want to deport you in the country you have illegally entered, or try to get into another country illegally, you can be detained. It's just that the French aren't doing it. And all those trying at Calais are breaking the law anyway.
No, look at the facts of this case. This was a Ghanaian woman who tried to get into the UK with a false passport. The French were not able to hold her legally after she was caught
Per the Telegraph 7th June
"People can only be locked up if they later fail to comply with a deportation order, if letting them go risks undermining the deportation order, or if they are convicted of another offence."
This means migrants can make repeated attempts to get into the UK illegally as a result of this EU ruling