This is hilarious, and I can't believe the discussion has gone on for 5 pages! At least there is definitely a consensus -- OP, you were not at fault in the least.
I do think the mom comes across as a twit, but I wonder if, as some folks have mentioned, she was just having an "I'm a terrible parent" moment and it somehow came across as blaming you. I've definitely had those times where my child is upset/injured/crying over something that I feel like maybe I could have prevented (however unrealistic that possibility). Mommy guilt is far reaching and irrational!!
Now that I think about it, until about a year ago, my young kids would not have been familiar with bar soap - I use bodywash, we have liquid hand soap in all the bathrooms, and we would have used liquid soap to bathe them as infants. It's only in the recent past that we've switched to bar soap at bathtime. Even in public bathrooms, most have one of those foam/liquid dispensers.
On a final note, this story reminded me of my 2-year-old this morning, who picked up a bottle of pink shavng cream from the side of the tub and, looking at the picture, asked me if it was ice cream!
Thankfully I told him no before he tasted it!!