@lelepond
On the one hand, I am fine with paying more, or paying for the whole bill (like if it's just small-ish - me and a mate at costa and it costs £8,) but I don't like people paying for ME, and I don't like owing anyone anything.
It comes from a sense of pride, not wanting handouts or freebies, not wanting to be 'beholden' to anyone, and having worked solidly and earned my own money for over 30 years.
Not saying I am better than anyone who HASN'T been a paid worker for X amount of years, I am just trying to illustrate that I HAVE never, and WILL never take anything off anyone and not return it, including money (or anything they have paid for IYSWIM.) because I have always been self sufficient.
As I said, I don't mind paying for OTHERS, but don't want them paying for me.
Maybe the MIL of the OP is the same. Nothing wrong with it. All people are different. Like @stripesandwings I pretty much disagree with most people here.
If someone really wants to pay their way, - even if it only a POUND that they owe you, it is MASSIVELY disrespectful to refuse to let them pay what they owe. You are shitting all over their views, their beliefs, and their wishes. Why would you DO that? Why? 
lelepond
When I first met dh I found his tit for tat attitude extremely tedious and embarrassing at times. Over the four years we have been together he has very much come around to my way of thinking.
WOW! What a nice thing to say about your husband. Why did you stay with him/marry him? 
Or did he change his ways to 'please' you?! (Or shut you up!)
And I expect his family find YOU 'tedious' and 'embarrassing' at times! 