I agree with the poster that Sony is not doing this to developers, that's a standard industry practice. The problem is that Sony is just creating more hurdles for developers to jump through in order to get their content to the Sony fanbase.
Many developers will see this as too much of a hassle and not bother with the PS3 at all, especially considering that the 360 fanbase is large enough to support most games. There are execptions to every rule, however, and I'm willing to bet that the company mentioned in the OP thought their game would do better on the 360.
Also, if a game is not selling well on the 360, why would the PS3 want the exact same game? At least adding content is better for marketing and maybe it would help sales on the PS3 as well. A game being cross-platform doesn't necessarily equate to more sales, the game still has to be good

.