Yes, the player aids for the main game were poorly set out, fortunately the player aid in the first expansion (Mission Dreadnought) was clearer. I can't find a copy of that card here on the geek so I'll try getting my old, old scanner to work. In the meantime you can point out to your friends
Page 14 of the Space Crusade Rule Book wrote:
Instead of firing, a miniture may attack using hand-to-hand combat either before or after moving.
Ergo if a miniture has the ability to fire, then they can attack in hand-to-hand combat.
If your players are still giving you hassle over this, point out that nowhere in the rules does it state how much armour a marine has
(Again it only says this on the reference card)
edit: this picture should make things clearer for your mates. It shows the HtH dice separate from the Heavy Weapons.http://www.boardgamegeek.com/image/165813