Because people who play that franchise will enjoy it, and even others who recognize Snake but never played a MGS game (like me) appreciated the inclusion.
So? Just because some people like it doesn't mean it's justified or logical.
I've always liked easter eggs, and it seemed like a huge, blatant one to me. I liked that. Why would Call of Duty put the same teddy bear across most of their games instead of putting something more realistic there? Why would Totaka put the same tune in all of his games? It's not a very good melody; why not substitute a better sounding song in its place? Because it's fun, and that's what games should be about.
Are you seriously attempting to say that not having pointless Easter eggs in this game would make it not fun? That is completely ridiculous.
I didn't even see it like that, but you're right. Having "Playstation" in the title would make the inclusion of non-exclusive characters seem wrong. At the same time, Smash Bros was never called "Super Smash Nintendo Characters".
... And they included non-Nintendo characters. Okay. Great. That's fine.
Before you go off on me saying that, notice that I said I thought it was stupid. I didn't say they shouldn't have done it. I said it was stupid because they had traditionally had Nintendo-specific characters, but I didn't say it was wrong or it shouldn't have been done that way.
In my opinion, however, if you're naming something to the effect of "OUR FRANCHISE CHARACTERS Fight and Stuff," it should be your characters. You want Easter eggs? Go punch in the Konami code on random websites and have a blast. Or call your game something that doesn't restrict your character pool.
And even then, a game could still have Easter eggs without putting non-company-related characters into their roster. Use a trophy system like Melee did and have cameos from Mario and other special characters. No need to make your supposedly company-specific roster showcase a character from another company.