I'm afraid there are times when, to protect the integrity of the whole operation, it is necessary to detain passengers on board. It is also illegal to detain passengers onboard. If Ryanair says it is for legal and security reasons, then that has to be accepted unless proven wrong. I'm afraid these days, there are additional security considerations that were not needed a few years ago- nowadays passenger lists are sent on ahead, and it may very well be that it is true it is a security requirement that all passengers are transported to the original destination airport. But it is also illegal to detain people unwillingly! Sort that one out. But once people start leaving, and having to get their baggage from the hold, you then have to cancel any further progress on the flight for several hours- it is not fair to the other passengers.
Years ago, we had to weather divert into FIU with a load of non-English speaking Italian passengers who were going to transit LHR for FIU. They were overjoyed- this was their real destination! Italian immigration said 'Non!- not allowed to enter- take them away!', probably as 'punishment' for not flying Alitalia- we didn't have traffic rights from Africa to FIU. We had to physically bar the door to stop them getting off- they couldn't understand, we couldn't release them. You can imagine the shouting.