Except... Fellay, Williamson, Tissier de Mallerais and de Galaretta weren't excommunicated for holocaust denial (and there's no evidence that the other three hold those views). They were excommunicate for accepting consecration as bishops from bishop Marcel Lefebvre, a man who was specifically forbidden from doing so. By accepting consecration, they automatically incurred that censure.
The Society of St. Pius X, which they belong to, has been and is suspended from practicing as priests. What has not
happened is that they are not
accepted into the Catholic Church again yet. As Jae pointed out, the group in question denies the validity of the Second Vatican Council (1962-65) and that situation must be resolved before they can be fully accepted into the Church.
What has happened is that the excommunication, the specific penalty incurred automatically upon their consecration by Bp. Lefebvre has been lifted. The Pope has not apologised, since neither he nor his predecessor either did anything (the penalty was automatically imposed by the actions taken), or allowed anything to happen that was contrary to Canon Law. They may once again legitimately receive the sacraments - i.e., go to confession, the eucharist, etc. Previously they were forbidden. They have not
been returned to positions in the Church.
The four bishops are still bishops because they were ordained validly, that is, according to the correct manner, despite the ordination being gained illicitly, i.e., without a Papal mandate (in this case, specifically against being forbidden to do so).
A couple of years ago, the leader of the S.S.P.X., Bp. Fellay, asked for two signs of willingness from the Pope that they could return to communion with Rome - one was allowing a wider use of an older form of the Mass, and the other was the lifting of the excommunications.
About a day before the excommunication was lifted, however, an interview was released with Bp. Williamson chattering quite happily about his theories on Swedish TV. It seems no-one picked up on it in Rome, however, and in any case, it didn't really relate to the canonical offences committed by the man. The excommunications were lifted, his superior apologised, he apologised (though not for his views, but the letter makes interesting reading in light of the latest business) and finally he either stepped down or was sacked by the SSPX (it's not entirely clear which, further muddied by said letter).
The bishop's views have not been endorsed, any more than any other bishop's political views are endorsed, held binding or approved (i.e., not at all). Nevertheless, the bishop seems to believe ahistorical rubbish extremely detrimental to all, and which makes dialogue with Jewish groups strained at best (several have threatened to break off ties with the Vatican altogether over this). It would be foolish to deny that the situation is delicate, and it has been made clear that the bishop must repudiate his views if he wants to function as a bishop within the Church.
While his private views are obviously his own business, the risk of him attempting to preach this (which is in any case beyond his authority) needs to be removed. At the very least, allowing him to influence others with his views on the subject would be unwise. This is probably what the current state of play is about. Nevertheless, you can't expect the press to catch subtleties.
Confirmation of Bp. Williamson's removal (French language)
, Bp. Williamson promises to apologise if wrong
, clarification from the Vatican in L'Osservatore Romano (Italian)
, Bp. Fellay denies such views himself
, Bp. Williamson's letter
, Bp. Fellay forbids Williamson to speak on political or historical matters
, 1988 decree of excommunication
, the interview in question
Apologies for the huge dump of links, but this issue needs to be understood clearly.