US senate passed extension of sanctions on Iran. Assume that it become a law.
Is it against JCPOA?
What is arguments of both sides?
What happened in the bill mentioned in this question is that the authority for sanctions was extended. This was consistent with the deal, which said that if Iran were to break the deal, sanctions would snap back. If the sanctions were unauthorized, they couldn't snap back until the authorization was passed again. As such, one could argue that the deal required an extension of sanction authority so as to enable the snap back provision.
It's worth noting that Barack Obama signed the bill under discussion. Source: CNN. Since Obama supports the Joint Comprehensive Plan Of Action, it would be inconsistent for him to sign a bill that counteracted the deal. He could have vetoed the bill.
The new Senate laws do not conflict (for now) with the JCPOA.
First of all, what the Senate passed is a law that allows the POTUS to enforce the sanctions. If the POTUS or Secretary of State want to reenact these sanctions, he no longer needs the Senate to agree, but it remains at his discretion.
It seems mostly a just a declaration of intentions, since it is difficult to foresee a situation in which the POTUS wants to put forward sanctions against Iran and the US Senate prevents him from doing that. Just a "friendly" reminder to Teheran about the "stick" of sanctions, and a way of political gesturing while allowing the JCPOA to continue.
It is worth noting:
a) As part of the JCPOA some sanctions have been lifted but there is still a control of Iran imports, requiring the UN Security Council agreement for the acquisition of heavy military hardware.
b) The USA continues to maintain sanctions against Iran. But now it just affects direct relationships between Iran and the USA, and third parties (v.g. EU, Russia, China) may trade with Iran (as long as not with items forbidden by the JCPOA) without being targeted by USA sanctions.
c) In a more general tone, as a sovereign country the USA is free to decide to which countries it wants to trade and with which countries it does not want to trade. The JCPOA does not force the USA to accept trade with Iran, so any sanction to that effect will not conflict with the JCPOA.
Yes, the law passed - even if only empowering the president to impose sanctions - violates article viii of the Preamble of the JCPOA, which states (emphasis mine):
The E3/EU+3 and Iran commit to implement this JCPOA in good faith and in a constructive atmosphere, based on mutual respect, and to refrain from any action inconsistent with the letter, spirit and intent of this JCPOA that would undermine its successful implementation.