Its a simple matter of mass. These did not have enough mass to be more than a firecracker IF they were explosive devices.
So explain to me why we aren't having panic attacks over everything else left unattended in a public place that doesn't look like a bomb? Boxes, bags, cars...?
Unattended devices left by overpasses and other structures are always suspicious... and if they are found to have wires or circuitry... then they are treated as if they could be bombs. As for "mass", plastic explosives take very little mass to do big damage.
I asked in another comment: are we ready to let terrorist know that it's OK to plant devices under overpasses and other structures - IF THEY ARE MARKED WITH CARTOON CHARACTERS?