They have a strong motive to oppose the deal: the US will tolerate the fact that India hasn't signed the Nonproliferation Treaty, which the IAEA is supposed to police.
I don't like this deal because it makes us look inconsistent: Iran now has an excuse not to comply with the IAEA since we just rewarded India for doing the same.
The comparison between India and Iran is not accurate. India NEVER signed the NPT (along with Israel) while Iran did. So, India isn't breaking any treaties while Iran is going against an international accord that it accepted.