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.