Rolled up jeans is normal - if they meet the top of the boot or show perfectly then as soon as you bend your leg (for example to walk, or sit) they are going to create a gap, and you will look like a 13 year old who has outgrown his trousers.
Why would you want to cover the top of the boots? Boots look great as boots, not masquerading as shoes.
Finally, they are not restricting. Denim has some great qualities, an one of those is that it stretches where necessary, and softens with each wash (unless you are wearing raw denim which is an entirely different conversation).
No-one expects Americans to dress with any style, but do try to remain open minded.