I disagree with the blanket statement that disliking MIL is prevalent or encouraged in contrast to the mumsnet position on children’s partners.
Plenty of threads are serviley pro parental prerogatives and female posters, writing about their experiences with their husband’s family, are routinely excoriated for failing to mend or care for that relationship, for being presumptuous when wanting more (or less) contact or help.
I'm with you, @pikkumyy77.
There's a prevailing, not to mention misogynistic assumption that this is wifework and that the responsibility for maintaining harmonious relationships with both sets of in-laws falls to the woman alone. Examples of this are clear upthread. The DiL wouldn't 'let' her husband see his family. What sort of wet lettuce would a man have to be to obey such injunctions? Men are capable creatures; they have individual autonomy; they are more than capable of maintaining a relationship with their family without DiL's input. My own father, brother and husband managed this perfectly well.
I looked at a board on the theme of estrangement. Overwhelmingly, where familial relations had broken down, it was the nearest woman who bore the blame. I suspect there are varying reasons for this. One is the expectation as to women's place. Another is that, if the blame for the breakdown wasn't pointed squarely at the son's wife, then his family might have to revisit their child's upbringing and ask some difficult and painful questions about their own place in the dynamic. Much easier to blame the newcomer.
Complete estrangement is a drastic measure; not one, I suspect, ever taken lightly. Where it happens, the estranged parents almost always say they don't have a clue what they did wrong. They were loving parents and one day, inexplicably, their equally loving son simply cut them off without a word.
This just doesn't ring true. It might be in some rare instances, but I very much doubt it's universally the case. There seems to me to be a strong element of denial entering the mix. Or annoyance about how the family's time is spread between relatives, without recognition of the fact that relationships differ and one size does not fit all.
Who's to say whether this situation would improve if responsibility were redirected to where it belongs? Sons' relationships with their family of origin are their own domain, not that of their wives.