This fashion rule/style was a little confusing to me at first. Parisians, for all the rumors of how much they hate Americans – have embraced A LOT of our fashion “lows”. Yes , sneakers are a low..
If education serves me correct – back ‘ in the old days of dinosaurs ‘ , you would go to school in shoes, 
then change into your ‘ gym clothes’ , which consisted of some ugly jumpsuit , & sneakers !! I can still recall my gym outfit – a strange poly blend of light blue horizontal strips –very similar to prison wear.. (which some girls will wear in the future). My sneakers were KEDS, blinding white color with little white socks. The boys wore cotton Converse high tops, also white. This was my first introduction into sneaker world.
Fast-forward to the 1980s, and sneakers became a fashion statement. Women wore these to the office, then switched over to pumps inside the work building. The Fashion World started to change the designs, and colors, and advertised to the public. 
Then somehow .. it became a common thing to wear everyday ..sports was no longer a prerequisite. Now wearing workout clothes, were ok on the common streets. France did not follow this same path.
In Paris, I watch how the invisible fashion line has been drawn, and its very visible – once you start to look for it. Sneakers are worn here, when you are still in puberty, (for some- this can go well, into their 30s). Allowed when you are doing a sport – like jogging, tennis, or bicycling. That is it! Otherwise, men are to wear a tie shoe( think like a saddle shoe)or a loafer, and women wear a ballet shoe.
Now here comes the Americans – yes, we stand out. 
For some reason – (and I may have been guilty of this too) we feel, whatever we wear back home, will be acceptable in Paris. When the tourists are on the Metro, wearing jeans – and sneakers..it looks sloppy – if you just change the sneakers to the ballets (don’t forget the inserts)– it looks more dressed up.
I have adopted this rule – in my OCD overdrive. I have not counted, but I think, I have more 20 pairs of ballets – staying in the B- rule too! (Black, Blue, and Brown & Beige). 
Some have a small wedge heel but most are flats with some kind of special toe design. You can spend very little or a lot on this kind of shoe- it fits every budget.
So if you decide to visit Paris, pack a couple of ballet shoes – ones for dressing and one for walking, (you will need the inserts for all the cobble stones).
Stay Safe
DIXIE in PARIS 