For me its honestly mostly semantics to say I am bipolar.

If you wanna delve deeper, like DHX said, saying "I have" seperates you from your illness, and so maybe thats the main reason folks are passionate about saying "have" they don't want to "be" the problem, so seperating the 2 is helpful.

I don't feel diseased or disordered, I tried on that illness label and it just didn't fit, I am quirky because my brain is wired differently. So I am bipolar because I'm wired differently

To me, its really not an issue whether you wanna say "I am" or "I have biplar", as long as you're not saying "I are bipolar"... Yeah that would bug me