I]Why would anyone take lascannons when there are so many options that aren't one shot only?[/I]
Lascannons are often available on cheap plentiful platforms, or on platforms that are good value and don't have an anti-armour weapon other than the Lascannon. Sometimes, you gotta be able to smoke some Necrons or Leman Russes and the ol' trusty quantity over quality can really struggle with that. Lascannons are neat because they let you kill things like Annihilation Barges in one shot, while an Autocannon or Missile Pod can spend a whole game trying to plink off Hull Points.
How can you use a fairly static army to shape an enemy force into doing what you want it to?
Sit on the objectives, or in gun range of the objectives with your own fast units to claim them in turn 5. To win the game, it doesn't matter who wins, who dies, what unit has what strength gun, what special character is the sexiest - all the game boils down to is sitting your mans on little points and stopping your opponent doing the same thing. See my example on offensive/defensive play - if you're on the objectives, then your opponent isn't, and if your opponent isn't on the objectives then he has
to be the offensive player. If he's the offensive player, you're the defense. Savvy?
How would you encourage players to tailor lists to the opponent - i.e have a good fun game for both players - if you ran a store?
Don't play pick-up games with people you've never spoken to before. That's the number one rule I've found (originally established as an Immutable Law of Playing Apocalypse, but later broadened to cover all 40k and Fantasy). If you've never met someone and talked to them, and they turn out to be a competitive player, then you only have yourself
to blame if you don't have fun in that game. This is a social hobby - talk to people beforehand, establish what kind of game you want to play, and work from there. I don't believe it's a store managers place to tell people how to play their game - if someone's playstyle and general manner is unacceptable or bad for play, the community will soon make that known.