The answer to the question in the headline is generally answered as “The U.S. supports the People's Protection Units (YPG) because it uses it in the ground offensive against Daesh.” Whereas, when the “Arab Spring” started in Syria, the U.S. had made very harsh comments stating that the Bashar Assad's administration needed to resign.
The U.S.'s aim was to topple the Assad regime. However, the U.S.'s current Syrian policy is not about the toppling of the Assad regime, but instead about strengthening the YPG.
The U.S.'s policy is now evolving toward granting Assad dominance in a certain region. We should also note that Assad is no longer a target for the U.S. Assad is now supposedly replaced by Daesh. Daesh is being used to hide the YPG's armed justification. It is clear that from the very start of the course of developments that Syria's division was planned, that a "statelet" is desired in the north of Syria with an exit to the Eastern Mediterranean. The YPG is an apparatus used for this statelet.
The above developments indicate that a large corridor starting from the Kurdish region in the south of Iran, enclosing Northern Iraq and reaching the East Mediterranean from north Syria. It isn't easy the establish a new state. You need time, land, populatio, but most importantly you need effective armed power. This scenario naturally threatens the territorial integrity of Iran, Turkey, Iraq and Syria. Therefore, these countries need to be “problematized," distracted, weakened economically, politically and militarily and compressed from inside and out. The Gülenist Terror Group's (FETÖ) coup attempt should be interpreted in this sense.
Iraq was literally divided into three parts after the U.S. invasion. The case in Syria is more complicated as everyone is fighting against each other. The U.S.'s Syria policies are advancing in this respect. Fighting against Daesh is giving the YPG the upper hand. As a result of Turkey's objections and counterattacks, the U.S. is promising an official rhetoric in relation to the YPG remaining east of the Euphrates. This statement of course hasn't convinced anyone. YPG forces, under the guise of the Syrian Democratic Forces (SDF), is moving toward the west of the Euphrates. The U.S. is trying to stop Turkey from preventing the YPG. Yet, the U.S. is overlooking the fact that this is contradicting with Turkey's perpetuity.
The covers are different but the organization is the same. U.S. Ambassador to Ankara John Bass said that Turkey's suspicions are groundless. According to Bass, Washington doesn't support an “autonomous” and “independent” Kurdish state. In one of Agatha Christie's novel, the killer is seen in different places wearing different wigs. The detective in the story describes the killer as “the man is the same, yet the hat is different” and solves the murder. The U.S. uses different names for the organization expecting to deceive everyone with their Syrian policies. Well they actually aren't trying to deceive, no one seems to be fooled by this one. You would have to be stupid or blind not to understand where the developments are taking the issue.