Why do my girlfriend's parents think I'm such a bad guy?

We have been together for over a year now and not once have we had a legitimate argument and we are very much in love. I'm 18 and she's 17 and for some reason he dad hates me guts(he is probably the cruelest human being on the planet, he's boarder line abusive to his children but that's besides the point) and he mom doesn't trust me. She knows nothing about me but she's come to the conclusion that I smoke too much weed and treat her bad.. I smoke weed on occasion, but my job drug tests so that's one reason why I can't and two I just choose not to. Her parents never include me in anything such as birthday parties, or barbeques and they never let us hang out and her mother always makes sexual puns about the two of us and I am getting tired of how they treat me. sorry about the rant but why do you think they treat me this way?