OK. So I freely admit this could all be conjecture on my part, but I have observed similar before.
I'm betting MIL didn't say much at all, particularly if the DH was present...Just let OP express rather angry amazement that she'd done it. If she'd done it out of love to be useful, that would already be apparent in her overall attitude to OP and she would freely admit why she did it and be at least a bit sorry and OP probably wouldn't have needed to post.
I think she saw a piece of OPs work that she'd put time and effort into, was perhaps a bit jealous in how it compared with MILs efforts in DH's childhood school holidays and thought... "Well that's coming down."
It keeps up the "harmless little old lady, just trying to help, look how she speaks to me" vibe whilst not acknowledging any fault at all... or not really explaining why she did it, (so there could be a real discussion and solve the matter there and then and move on) -which also looks like MIL is "rising above it"
How very Royal Family.
It then becomes harder for OP and DH to work out what to do, because they don't understand why she did it ( just look at this thread we're all puzzled) which irritates them, and because the real why of it is so unclear and so frustrating, OP is very cross about it... (as intended). OP starts to look like the aggressor of the piece. Which becomes even more annoying.
The MIL is effectively going into another woman's house and making her mark/imposing her will, in a way that will cause disruption and annoy OP because OP will complain to DH and start (MIL hopes) to look like OP's being mean to MIL, or OP can't get on with her etc. Essentially that its OP's fault. MIL may not even realise she's doing it, just thinks shes asserting herself.
Or maybe I'm reading too much into it 😀