Probably to scare you out :) These seem to be the Souyz debris (green/gray painting), not too dangerous I imagine. Proton-M remains are completely different story though.
Wouldn't the harmful stuff eventually dissipate? How long would one have to wait to safely scavenge a booster sitting in the open?
It's not possible to safely scavenge a booster sitting in the open, because by the time it could be dissipated it will long be scavenged unsafely by someone else.