Here's why.  My sister and her family were unbelievably "toxic."  Yet year after year after year after year I stuck with them.  I loved my nephews, one in particular.  After our mother died, and during my first "battle" with cancer they dumped me.  I guess (because I'll never know) it was because I was so toxic.  There had to be a better way.  I am brokenhearted over this and it will never be healed.  So, who's the toxic one?  Everybody, I guess, depending on who you ask.
Yes, there are clear cut cases.  Then again, not always.  It's an easy claim to make and it makes a lot of folks feel better when they unceremoniously get rid of someone who may need them.