Why are people with STDs shamed?

Most likely because they feel that STDs are always the own person's fault or feel like they could get it too.

People with stds are seen to be people who 'fool around' with random people, getting into one night stands (for males) and is an identity for a 'slu_t' (for females). Such stereotypes ignores the fact that one could be careless or other factors such as not keeping clean, etc, and blames the "already victims of STD" for their diseases.
I consider avoiding them a form of 'shaming' too. Why do people avoid STD victims? Stereotypes make these STD victims be seen as irresponsible and furthermore, STD can spread and nobody wants to contract that 'dirty disease' (what many people think of STD as).
However, these stereotypes are true to some extent. While some people could be very unlucky, most of them get STD because they are irresponsible and 'fool around', meanwhile spreading STDs everywhere. Because of such people that's why the public tend to look down on these people.
You may say it's none of their business, which is true, but prejudice is ingrained (it's a mindset) can never be removed.

Cause they didn't wrap the weenie, and it was their fault for not wrappin' it up.

Its a shameful thing when you could have prevented it. For one reason or another.

don't know

I have never come across shaming and have no idea what you are talking about.

> It's not like people want to have STDs or anything

Yes, I believe that like hell. Neither would you want to have the plague or cholera. And yet, **** happens. Shaming people with STDs comes from people who really think that it could never happen to them. Or from ignoring facts; STDs have a wide range from vaginal fungus that can spread onto a man's penis (had that once, was treated within a week) up to the more deadly ones. Which I suppose aren't that funny, but still I don't always take the recommended precautions. I visit my doctor every half a year and think I'm medically clear, but, yes: SOME risk always remains.

Cause STDs are ******* gross

Often, it is not the STD itself that is actually bringing the shame down--it is the belief that someone can only get STDs by being careless or promiscuous. This is fallible reasoning, however. For one thing, some people are not forthright about their own sexual health. In the case of herpes, for example, the condition is not easy to detect outside of an outbreak (OB). A person may assume there's little risk of transmission outside of an OB, and therefore not divulge their condition to avoid shame. Some may not,even know they have an STD. Even when a person with an STD is forthcoming and takes precautions, condoms can break. Partners can cheat. And rape victims don't have a say--and rapists care little for their victims' sexual health. Also, STDs can be passed through tranfusions, birth, and other non-sexual contact. So while society may shame those with STDs, the line of reasoning with which society may justify this is flawed. And it perpetuates the problem as shaming leads to hiding.