Throughout American history, the flag of the United States has meant different things at different times. It is thus a reflection of the cultural values that are in circulation at a particular time.
One can argue that the meaning of the American flag became considerably more fluid in the 1950s when the image began to be used in non-patriotic ways. The painter Jasper Johns appropriated the flag as an aesthetic device, notably, with the resulting series of paintings more ironic than celebratory. Those works foreshadowed the pop art movement, which co-opted iconic American imagery as creative fodder and social commentary.
This tongue-in-cheek or wink-wink interpretation of the American flag intensified in the late 1960s when it was adopted by the counterculture in a satirical or humorous way. Hippies used it on clothing and elsewhere as a critique of consumer culture and to poke fun at how conservatives viewed it as sacred. American flags, along with draft cards, were also occasionally burned at Vietnam War protests. None of this would have happened during or before WWII.
During the more conservative Reagan era, however, the flag regained much of its traditional values. The Cold War was still in play, so the flag was used as a primary symbol of the American way of life in contrast to Soviet communism. (The repeated chanting of "USA" during the 1980 Olympics, especially during and after the United States vs. Soviet Union hockey game, also reflected the resurgence of national pride.) Through the 1980s and after 9/11, marketers adopted the semiotic value of the flag in a big way, knowing it was advantageous to "patriotize" their brands given the pro-American cultural climate.
Today, against the backdrop of the Black Lives Matters movement, all kinds of symbols are being reexamined in order to determine if they promote or endorse racism in some way. As the most obvious symbol of our national identity, it's not surprising that the American flag is part of this closer scrutiny being paid to racial inequalities, both past and present.
Although the meaning of the flag is up for grabs depending on one's political slant, it appears that the right has adopted it as part of the "Make America Great Again" ethos. This is reminiscent of the "America, Love It or Leave It" bumper stickers one would see in the late 1960s as a reaction to the countercultural movement (and especially to the protesting of the Vietnam War). Today this kind of nationalistic use of the flag seems to be a response to multiculturalism and globalism.
From this perspective, it can be understood how the flag can be seen as representative of white (and male) power. Until relatively recently, not only were white men dominant in virtually all aspects of everyday life in this country, but many took active steps to repress potential threats to their power. The flag can therefore serve as a reminder of how the nation has fallen well short of its noble ideals.
It can't be questioned that the United States has a racist past and that African Americans are still economically, socially, and politically marginalized. The flag can thus symbolize both America at its best—our inalienable rights grounded in democracy and freedom as brilliantly expressed by the Founding Fathers—and the institutionally embedded racism that is an undeniable part of our history.