On the matter of why it was not allowed, encyclicals such as Mortalium Animos seems to suggest something greater than just sin. It was viewed as potentially leading to indifferentism and the founding principles of such gathering as being threatening to the Church. MA suggests that such meetings, after leading indifferentism, then leads to agnosticism and atheism.
Then if one were to read MA for an example, and then look at this event, would not one act out of zeal as the SSPX had done? They would not be thinking of simply avoiding sin but perhaps larger matters. If they had lost children to other faiths, this feeling may even be more pronounced.
At this juncture of things, one could say that the SSPX are being disobedient to the Bishop. But in a time where incorrect practices had been frequently and commonly allowed by Bishops and priests and later condemned by the Church, there is an issue of trust at play.
Even I, though I am not SSPX, sometimes feel a mistrust toward my own local bishop and the events (not particularly speaking of ecumenical events here) he wants to hold. Always, upon learning of an event, I would find out the details and just look up Church rubrics to see what she says on the matter. Every-time I discover that the events are legitimate, my trust in him grows. But it also is shattered every-time I find out that the event has dubious foundations or that it has been warned against by the Church.
I think this is unfortunate that it has come to that. But it is just where we are today. There is an inability to trust the shepherds in the Church because they had let down the faithful 60 years ago in a rather widespread manner. For those like the SSPX, it seems that they have come to associate this let down with Vatican II and therefore tend to look at what was said and done before Vatican II to judge the actions of the Bishops.
In their eyes, these gatherings can seem very much like sacrilege and something to be resisted. Perhaps they see themselves like those in the times of the Maccabees.