| I'd imagine that the rules are in place for safety reasons than comfort.
Jeans: restrict movements somewhat. Let's say you're doing squats and suddenly you can't lift it anymore because your pants are tight. Well, someone decides to come to your rescue and because of your jeans restricting your movement, you fall over board and all that weight comes crashing down.
Or let's say you decide to hop on the treadmill for a bit. Since you're wearing jeans, you can't move and you trip and crack your head open on the spin.
Cargo Pants and other things with lose pockets/items: Safety hazard. Woodshops and auto shops don't allow these for a reason and the same for the weight rooms. These things can get caught on something. Logic concludes that the less things hanging out and the tighter your clothes are, the less of a chance it will catch on to something.
Let's say someone was lifting a really heavy bench, and you just happen to pass by and your pants get caught on the bar for just a second and causes them to lose grips and smash their chest.
Or let's say you're walking and your pockets get caught on a weight rack and it comes crashing down on your foot.
If I ran a gym, no one would be wearing lose clothing, sandals, or jeans. As a matter of fact, just to show them how unsafe it is, I'd drop a five pounds on them.
What I recommend to do is use a cargo pants, but cut out all of the pockets.  | 
