I remember reading about this. What I understood was that at lower heart rate your body burns more of a mix of fuel (burns both glycogen/glucose and adipose tissue). The theory with this is that to burn fat/lose weight, you work out at a heart rate of between 50 and 75% of maximum to keep your body using fat for energy. I think at the higher intensities beyond 75%, you still do burn fat, but at that level the demand for energy to muscles requires the fastest energy breakdown system your body has (they call it the Kreb cycle), which is breaking down glucose molecules into usable energy called ATP. The energy breakdown pathway for adipose tissue is slower, so your body relies more on the glucose/glycogen energy pathway and less on fat burning when it's working out that hard (past 75%). The new understanding of lactic acid is that our bodies do use it for fuel, it isn't just some breakdown of respiration/waste product that occurs because the body is pushing exercise faster than oxygen can be provided.
I still read regularly that lower intensity workouts of 45 minutes to an hour are great for fat burning (like a long bike ride at talking speed). I'm not sure why your trainers want you to limit it to 30 minutes. That may be that they don't want your body to possibly burn muscle that they're trying to help you build up? There is also the understanding that the more lean muscle mass our bodies have, the more energy we burn both at rest and during our daily activities. So they want you to become the most powerful "furnace" they can build you into by protecting your muscle mass that you're working so hard to build.
edit-sorry, I forgot I was in the Ask Cathe forum.