The shrapnel thing really trips my "that doesn't seem right" intuition WRT how explosions work, unless those bins had some really unfortunate design features. I believe someone official may have said that at some point and it may have been picked up in popular culture as a reason, but it strikes me as kinda "truthy", pending more info—I'd expect a container of that sort, especially with a bit of a gap between most of it and the device, to tend to break into a small number of large pieces with terrible aerodynamics, much of it bound generally upward and slowing a ton before reaching human-height again (assuming the bins were fairly big and the bags/bombs tended to end up at the bottom or middle), making shrapnel that's at best (well, worst, but you know) very marginally more effective than the bomb sans same shrapnel, if at all. Shrapnel that's big (tending to strike over a large surface area) and rapidly-slowing is fairly useless shrapnel.

The explanation that they were a good place to hide a largish bag so no-one would get suspicious about them seems entirely sufficient to explain why they were used, and why it might be a good idea to remove/disable them and replace them with something you can see the inside of.