I think that not increasing their crit damage was an intentional decision, rather than an oversight on the designers' part. I personally would be really quite annoyed if a solo did a big attack, got a crit, and killed me through no fault of the party - critical hits are still nasty whenever they happen, since they still do a nasty amount of damage that you weren't expecting. It doesn't really affect their overall damage output, since critical hits only happen 5% of the time, but it does make it more likely for PCs to get unexpectedly splatted. I don't see this as a good thing, really.