I understand that double pole circuit breakers feed 240v (two120's) to
water heaters, stoves, ect...

But, Im confused about the amperage marking on the middle tab; I see
that most say 30A. Does that mean the each pole from that circuit
breaker can handle up to 30A each pole? 30 + 30 = 60A total? or is the
amps on each hot leg split into two; 15 amps each pole; 15 + 15 = 30A?
I never worked with 240V circuits before, obviosly. I just need to
install a small baseboard heater in my shop for the upcoming winters;
it calls for a 20A 240v breaker.

Another quick question, but off topic:
Why don't wire manufactures insulate the bare ground wire inside romex
cables?