Why is use of calcium carbide as an artificial fruit ripening agent banned?
According to Anwar et al. the use of $\ce{CaC2}$ is discouraged due to 


  dangers of explosion and carryover of toxic materials like arsenic and phosphorus to consumers, thus making the healthy fruit poisonous. Since no technical knowledge is considered necessary for its anomalous use, higher quantity of calcium carbide needed to ripen immature fruit, makes them 
  tasteless.


For references and more information, see: Anwar, R., A.U. Malik, M. Amin, A. Jabbar and B.A. Saleem, Int. J. Agri. Biol. 2008, 10, 35–41. Available at researchgate.net.