That is the delima with the BJP. As a political party, they must have analysed the issue very well, and know why left are opposing it.
BJP is also compelled to oppose the deal as our PM has not taken them, or for that matter anyone in the opposition parties into confidence before signing the deal. None of the members of parliament are aware of clauses, terms and conditions in this agreement. If the deal is against the interest of India, ireespecitve of what China feels or CPIM feels, BJP as principal opposition party has a responsibility to oppose it.
As of now both BJP and left are opposting the deal, but for diferent reasons! Left is oppositng the deal to appease China and BJP for national interst.