The thing is, whilst Citizens United does sound absurd on the face of it, the judges of the Supreme Court are not idiots. There are reasons that they rule the way they do--even the conservative ones, as much as we'd prefer to think they're unthinking dogmatists. Yet, there's never any attention to paid to their judicial reasoning: it's always the headline "Citizens United = unlimited corporate political spending". Again, bit cheap and easy, you know?
Especially because it's quite simple to understand why the court ruled as it did. If the First Amendment does anything, it restricts federal restrictions on freedom of speech of individuals and freedom of the press. Since corporations are simply an association of citizens (also guaranteed by that amendment) united in a purpose which is commerce, a logical extension of the First Amendment is they cannot be denied political speech. The First Amendment cannot distinguish between citizens (because otherwise it'd allow you to, say, ban a proportion of the citizenship from speech) so, if you accept that corporations are associations of people for business (which in your reply, you imply you do) then, since the Amendment cannot discriminate, then it cannot exclude political speech by corporations.
Finally, if you accept the spending money in advertising is a form of political speech, it follows quite logically that you must allow unlimited campaign finance by corporations (anyway, the law isn't as imbalanced as you think: it allows unlimited contributions by unions, too). So whether or not the Supreme Court was "right" is always up for debate (no doubt you think it was right in desegregating schools in Brown v. Board of Education, or guaranteeing a woman's right to an abortion in Rode v. Wade), but Citizens United is definitely a (if not the only only) "right" interpretation of the Constitution.