In addition to the comments before me, it's also because the rules are not as simple as they may seem, and the rules change with practice over time. Unless you

In addition to the comments before me, it's also because the rules are not as simple as they may seem, and the rules change with practice over time. Unless you believe that the U.S. Constitution and laws are absolutely inflexible (which not even the most strict textualists/orgininalist would, I think, contend), law changes via practice and via interpretation. In the US, it's almost always been the case that the lines between treaties, executive agreements, and something called congressional-executive agreements (consent of Congress, but less than the super-majority requirement of a treaty) are blurry and ambiguous. Given that, you could either presume that whatever the Constitution doesn't explicitly forbid, it permits (in terms of executive power), or the opposite. Most presidents have presumed the former in the realm of executive agreements and treaties, meaning that, because the Constitution doesn't say that can't make these executive agreements, they've felt empowered to do so. This has been affirmed as permissible by the courts, and has happened very frequently, although that's not to say that future scenarios might arise that test the boundary between executive agreements and treaties in a way that hasn't been pushed before. In summary: U.S. Constitutional law and the common law are very complex. It's not entirely clear that any rules are being broken, even if Obama does not ask for Senate approval. A court of law would be the proper place to resolve this, presuming the case was justiciable, which is another issue. Lawmakers themselves are no more informed than this comment or other commentators, meaning that they themselves can't provide any definitive answers.