Fruit Frenzy is unique with how features are triggered, in that it requires getting 5 matching symbols on a payline to trigger the feature. If you play the max (25 lines), you have 25 chances per spin to hit a feature (or multiples of the feature). If you play fewer than the maximum, let's say 15 lines, then you have 10 fewer chances per spin to trigger the feature (or multiples of the feature). The feature always plays all 25 lines, regardless of how many lines you play in regular spins/play. It's built that way, and doesn't change the RTP (Return to player) in any way. If you look at the "Help" screen, it clearly states that the feature plays all 25 lines.
Now, in the case of Bovada's version of Fruit Frenzy, there's a graphical error when the feature is playing, where is shows the same number of lines you were playing prior to triggering the feature, however, it's actually playing all 25 lines. I verified this by playing 15 lines in regular play, and when the feature played, it showed/paid wins on lines higher than 15 (when those lines had winning combinations). So, the theory that some places are "onto" this isn't true, as the game plays exactly how it was designed to, and there's no reason to think the casinos are using any manipulation of game play.