It's worth pointing out that there can be other reasons than player lazyness for giving a character amnesia, in fact, I'd say the following are equally likely justifications:
1) I am sick and tired of the 14-page backstories you have us write. If I want to be an author, I'll write a book.
2) Why the hell should I even bother with a backstory when you ignore them? Maybe if I was the one person in the group that you're hitting on, you might pay attention to my character history, but I'm not, so I won't.
3) Backstory? Why bother? My last three characters have barely lasted a session anyway. That's why I gave my character the name "Number 4".
4) I'm tired of you using my backstory as tools to torture my character. This time there won't be any taking the people and things my character cares about and destroying them- not only is he an orphan, he's an amnesiac orphan who cares about nobody and nothing!
Seriously, before accusing the player of lazyness, one might want to consider whether the player is actually saying something about the GM's style of running games.