It's either they don't want to draw the censored part or they want to leave it to your imagination. It does not really matter as our brains could probably fill in the gaps.
Uploaders don't censor shit, okay? Japanese law requires that genitalia be censored. The doujins you read that are censored, the original fucking artist did that. If it's uncensored, that means that some great fellow decided to bust his ass and uncensor it himself.