They are against it, but I don't know a specific "excuse." Many bishops seem to approve it only as long as only older people go. They don't want any young people there. In Hawaii, the bishop allowed an indult, but he made the petitioners promise to "keep it a secret." So, even if you called the chirch,they wouldn't tell you about it.
That sounds ridiculous. And, of course, there is no sound reason for bishops or anyone else to be against the Indult Masses in Latin.
People will tend to have different ideas about what they consider extreme, whether that's Opus Dei, Regnum Christi, the LCs, charismatics, EWTN, Ave Maria College, or whatever. Taking a look at organizations or movements which call themselves "Catholic" in itself is not wrong. Being fair, accurate, and keeping things in perspective is needed though.