I've been reading a little too into some recent Churchy news about the Pope "imposing an oath" on a Franciscan group I had heard nothing about. I don't want to cast aspersions on the Holy Father in the least, but what is the deal with the way the Franciscan Friars of the Immaculate celebrate Mass? How does and how can it differ from what's in the Roman Missal?
Good luck trying to figure that one out ;) Let me, though, attempt a little insight.
For one, is has been said that the FFI has not contradicted magisterial teaching on faith or morals. Rather, what it appears to be is that leadership and discipline has broken down significantly enough that some members of FFI itself have called on Rome to help them sort things out in 2012 (that's not, by the way, a complaint but a formal petition the Holy See has to deal with). The Holy See indeed dealt with it, through a visitation (=inquiry) that lasted a whole year and ended this July. It found a lot of shortcomings, in particular through an questionnaire in which over 60% of respondents noted that there are big problems in how the FFI was run. Hence the appointment of Fr Fidenzio Volpi as Apostolic Commissioner, who is somewhat equivalent to an Administrator in English insolvency law: he's overseeing where things will go (bankruptcy, reorganisation &c.) on a temporary basis.
The unfortunate thing is that we know all about what the administrator did, but never why -- those decisions are not, as a rule, published and thus everything is ascribed to a hostility to the Latin Mass. I daresay the FFI probably hasn't done anything liturgically wrong. Rather, the main problems seem to be at the head, in particular with the founder and Superior General, Fr Manelli. He has been rather absent from the whole picture, having admitted himself to a private clinic. To which Fr Volpi's reaction was 'okay then!', and thus forbade him to leave until he's recovered. This was then widely reported as "conservative priest who likes Tridentine Mass in house arrest by Vatican commissar!", giving the impression that a lot of this had to do with their particular rite.
One of the liturgical misdeeds they have committed was 1) the Franciscan Sisters (the female part of FFI) putting through a modification of their constitution under which they would be allowed to choose spiritual directors from those FFI friars who approve of the Extraordinary Form and 2) Fr Manelli mandatorily imposing the Extraordinary Form. There's a difference between the Extraordinary Form being 'permitted' and 'mandatory', and the vow (which is really unusual but on the other hand, given its content, is something any Catholic ought to be able to take with confidence) is meant to sort that out.
A little footnote to this is that liturgical conservatives feel a bit persecuted at the moment, and rightly so: there have been other hotspots of heresy and the Vatican hasn't tackled them yet. It's not entirely an unjustified feeling but every person investigated first will point to the other guy. Normally, interventions like this are staged when there has been a systemic breakdown of things, and as present in this case, there is a lot more to it than merely their preference for the Extraordinary Form.