Why are the Capulet parents to blame for the deaths of Romeo and Juliet?

Shakespeare's Romeo and Juliet is the
story of young lovers who create a relationship which their parents know nothing about,
get themselves in trouble by their own actions as well as the actions of others, and
commit suicide so they can be together forever. The premise of your question is that
Juliet's parents are responsible for their daughter's death, and there is little
evidence that Juliet's death is their fault.

If one were to
make the case as you state it, the Capulets' only action which contributed to Juliet's
death is marrying her off so precipitously. It can be argued that if Capulet had not
insisted on her marriage to Paris, Juliet would not have sought Friar Lawrence's help,
Romeo would not have thought she was dead and killed himself, and Juliet would not have
stabbed herself in despair.

The reality, of course, is that
both Romeo and Juliet--and particularly Juliet--act independently of their parents in
almost every way and are therefore responsible for their own deaths. Juliet chooses to
marry Romeo without her parents' permission or knowledge; if she had told them, she
would not have died as she did. Juliet voluntarily sought the Friar's counsel and
voluntarily took the potion--without even telling her former confidante, the Nurse.
Juliet sees her still-warm husband and chooses not to leave with the Friar; instead, she
picks up the dagger and chooses to kill herself.

The one
thing Juliet's parents did, though rather outrageous and hasty, seems rather small in
comparison to all the dramatic and drastic--and deadly--actions Juliet does on her
own.