You can re download Lion on all your authorized Macs using the same Apple ID for free.
How to re download apps from the Mac App Store:
Open the App Store. From the menu bar click Store > Sign In
Click Purchases from the top of the App Store window.
Select which apps you want to re download. Then right or control click where you see Installed then click Install.
Yes, it's legal. It's ok to install Lion on all your authorized Macs using the same Apple ID.