I don't think this is prejudice…

[Dystopia]

On birth certificates and ID cards, where it says "mom/dad" it usually means biological parents, so there's no reason to put legal guardians - If I'm adopted by my grandma, or I am living with her most of the time, my ID card will still have my biological parents' name because that's what it says in the title - Of course, legal guardians are important, but usually the law never gives 100% similar rights to legal guardians compared to biological parents - For example, my GF is adopted and while her adoptive mom had many rights, she still needed permission from her biological dad to leave the country and go on vacation. Biological connection is legally never irrelevant, or it is irrelevant in little cases.

It's one thing to list "legal guardians", but that doesn't make you a biological parent, and from my experience most ID cards and similar documents, when saying "parents" mean biological parents merely, regardless of who takes care of you and if you like your parents or not. My GF hates her dad, but still has his name on her ID card. It's part of the legal process.

Maybe in Portugal, but here, for example, my parents' names have never been on any of my identification except my birth certificate. We also have a closed adoption system where adoptive parents or adoptees have no right to even know who their bio parents were.

It's normal in family oriented societies. Birth certificates, from my experience, serve the purpose of proving that you were born, where, to whom and in which day, etc. I have my birth certificate, it says the place, hospital, city where I was born, my mom and dad's names, etc - Birth certificates are made right after you are born and filled right away, so it will always have the biological parents' names. BTW, there's a law that basically says you must have the father's name (this is to prevent fatherless kids because some dads run away from responsibilities) and if it is necessary the mother can accuse X person and you have to take a DNA test to prove you're not the father - If you refuse, it is assumed you are the father (why would you refuse the test if you know you're not?). 

Like I should have said, but forgot - It depends where you live, the laws in motion and cultural circumstances. ID cards are a bit different - When I mean ID card, it's basically what I used to sign up for A69, it has my name, age, height, parents' name, social security numbers, etc - I think it is possible for your bio parents to be prevented to have any contact with you, but it is not possible to NOT have your parents' name on your ID card, there's no way around it unless someone is conceived in a laboratory or something.

I'll say this - I think people should not be prevented legally from knowing who their bio parents are if they are adults and wish to know. It is a right as much as knowing your family tree, and there's certificates you can get - Even if it costs some money.