It's basically because the preview isn't limited to token gain. The preview, ideally, shows all numbers changed by playing a card. It could become more confusing if it always shows token changes but sometimes not damage or stock changes.
I'm sure you understand this already, but I'll put it here for overall clarity and for new players who may not understand the reasoning. Since there are many cards that rely on something in the opponent's hand, it was once possible to use the preview function to "snoop" at the opponent's cards. Imagine using
Sphinx with preview to see which hidden cards are zero-cost and which are expensive, then selecting the one that gives you the most stock. This possibility had to be eliminated, so preview was made unavailable in Hidden mode.
It would be nice if the keyword mouseover text could give this information. Presently, it tells you the values for playing the card and the bonus gain for additional cards. Could the mouseover text be coded to include a calculation of the gains for playing the card at that moment? I understand this would be adding a dynamic element to what is currently unchanging text so it could be difficult to implement, but if it isn't, it would put the information right in the place where someone would have to look to otherwise do the calculation manually.
Better yet, could this be added to the token counters at the bottom of the playfield? Like, mouse over the word "Alliance" to see "+20" when there are two in hand?