Is it normal my parents to downplay bad experiences with people?

My parents seem to add a major complication to my life every year. Every year they seem to involve themselves with a no good person, even let them live in our house. Usually in the spring til autumn. This makes me spend time and energy correcting/preventing the wrong doings of these people, and protecting my younger sister from their BS. They also loose money. When my parents get fed up (a long time afterward) and kick the crappy people out of their lives, A few months later they act as if the crappy person wasn't so bad.

"Oh they were depressed" or "you had a better upbringing than them, forgive and forget"
This concerns me a great deal, mostly because they make a similar mistake every year.
I'm already moving out but the situation just irks me.