Those criticizing the Indian government for the Sharm al-Sheikh declaration essentially want it to remain wedded to the least productive course of action: not talk to Pakistan, while simultaneously minimizing Pakistan's incentives to act; and all in a situation where India lacks the power to make the Pakistani government act as it wishes (it is unclear if any country could do so, but to the extent the US can, India is unlikely to be able to get America to exert pressure on its behalf without directly implicating the US in the Kashmir dispute -- a development India would be suspicious of, and has consistently resisted).
Second, the significance of the "concession" is itself being overblown, based on the theoretical possibility that the peace process would move forward, even if Pakistan took no action against anti-India terror groups. This is downright silly: where on earth would such a peace process progress to? Both the Indian and Pakistani governments have to be aware that no peace deal over Kashmir, Sir Creek, or the Siachen glacier, could be viable in an atmosphere of continued attacks in India emanating from Pakistani soil. The Sharm al-Sheikh declaration cannot change the political reality that no deal can be sold to the Indian public under such circumstances (the opposition outcry against the declaration might even be seen as confirmation of this). If the Pakistani establishment were to genuinely believe that the declaration means that the terrorism issue is off the table, it would simply be deluding itself.
Third, there is the question of hypocrisy. When in or (in the case of the Left) close to power, the opposition parties have done pretty much the same thing they now criticize the present government for. Indeed, it was the BJP-led NDA government that initiated the peace process with Pakistan -- despite the fact that no progress had been made for years in apprehending those responsible for the 1993 Bombay blasts; or the likes of Masood Azhar and Omar Sheikh, freed from Indian jails in exchange for the passengers aboard an Indian Airlines plane.