It's my private deduction that they know the legalities (or lack thereof) perfectly well. I'm not buying that people who are otherwise switched on, when it comes to both legal and political issues, genuinely believe that the moment your marry, your husband's name 'becomes your legal name'. I've heard this one more than once - of course it comes from women who have already been married, sometimes more than once - so they know fine well that your family name is YOUR legal name unless you take active steps to change it.
'Well, I respect MY husband' is by no means something I've never heard before, either.
I don't use a 'Mrs' title and I don't want to be addressed by such a title, or by a family name that isn't mine. Unfortunately my MiL is also one of those people who refuse to acknowledge the most basic, fundamental of courtesies to respect another woman's actual identity, rather than the false one she'd prefer to bestow on her.
She does it to annoy. Of course I could retaliate and address her by my FiL's name (they're divorced, but she has more claim to it given she's actually used it and I never have). But I'm not descending to her petty level simply to make a cheap point, or let her know that once upon a time, it used to piss me off. (We've been married 15 years). Instead, I rather pity her for not having the gumption to at least address her disapproval of me openly, rather than hiding behind cowardly gestures.
But I think the PP who made the point above was quite right. It's disingenuous, selective ignorance practiced by those who want to put women back in our box.