Why does it make the bones and teeth soft?
I don't have a clue. However "meth teeth" which are rotted or dark blue looking is not uncommon. I've heard of people pushing in on their skull/cheek bones and moving them. Meth is after all made of poison. Meth heads are unable to keep jobs because they simply cannot go to work on a regular basis.
A common sign of meth abuse is extreme tooth decay, a condition that has become known in the media as “meth mouth.” Users with “meth mouth” have blackened, stained, or rotting teeth, which often can’t be saved, even among young or short-term users. The exact causes of “meth mouth” are not fully understood. Various reports have attributed the decay to the corrosive effects of the chemicals found in the drug, such as anhydrous ammonia (found in fertilizers), red phosphorus (found on matchboxes) and lithium (found in batteries), which when smoked or snorted might erode the tooth’s protective enamel coating; however, it’s more likely that this degree of tooth decay is brought on by a combination of side effects from a meth high.
The above from pbs website.