I don't think you will get flak (not slack) for the question.
I don't think for a second that Jada Pinkett Smith and Will Smith are boycotting because HE wasn't nominated. (There are always glaring omissions.) I think they, and Spike Lee, and others, are boycotting because NO people of color were nominated anywhere.
Brilliant answer by JB below - BET and NAACP celebrate people of color, and ESPN celebrates sports heroes. Women's organizations will celebrate women of achievement. And yes, that's because these groups are under-represented in mainstream awards. So, no, white people aren't nominated for those. Women aren't nominated for "Best Actor" and cats don't compete in the Westminster Kennel Club dog show. Baseball players don't compete for the Heisman trophy and hockey players don't qualify for the Cy Young Award. My husband is a writer and will never, ever, have a shot at Teacher of the Year. If there are equally prestigious awards in a different category, that's one thing. But there aren't.
The sad fact of the Academy is, although there is a black president, the average make-up of it is 63-year-old white men. You could say the same thing about Congress. Or Corporate America. So while they might hire Chris Rock as host, that doesn't mean that the opportunities are even. The Oscars are THE awards for motion picture excellence. If there are only white people nominated, that means that no people of color were worthy, in the eyes of the Academy. And that sucks.
Whoopi Goldberg had a very good point on "The View" the other day. (And she ought to know - she was the first person of color to win a Supporting Actress Oscar since Hattie McDaniel for "Gone with the Wind" - even though Ms. McDaniel couldn't sit with the other nominees!) Whoopi's point was, it's not just about the Oscars and what you see represented in the nominations. It's what you see represented the other 364 days of the year in terms of movies, roles, directors, music directors, cinematographers, and so on. What are the movies we are going to see all year long? It's mighty white. But the MONEY is white too - producers, executive producers, directors, etc.
It's also overwhelmingly male. Women, particularly those over 45, are really scrounging and struggling for roles. Yes, ONE white woman over 50 might get a great role, but there's only one. Or two. So if Helen Mirren gets it, Maggie Smith and Judi Dench and Vanessa Redgrave don't. If Meryl Streep gets it, then Sally Field and D. Wiest and D. Keaton and Ellen Burstyn don't.
And it's not just the movies - all the late night TV shows are, and have always been, hosted by men. Joan Rivers tried, and did not succeed. But she's the only one who even got a chance.
There's also an ageism problem. Maggie Gyllenhaal is 37, but was told by a Hollywood producer that she was "too old" to be the love interest of a 55 year old man. In the last Bond film, there was a big firestorm over the "advanced age" of the Bond woman. Daniel Craig, to his credit, shot it down. And I quote: When the interviewer asked about Bond "succumbing to the charms of an older woman," referring to Bond's love interest actress Monica Belluci in "Spectre," Craig quickly shut him down. "I think you mean the charms of a woman his own age."
So there's plenty of healthy debate about whether or not a boycott is a good/productive/ healthy thing. It worked for the Montgomery bus boycott decades ago of course, it's working for the Barilla pasta products and Chik-Fil-A (anti-gay stance), it certainly worked with the Salvation Army (red kettle collections were WAY down around Christmas due to their discriminatory policies), and so on. Will it help with the Oscars? I don't know.