It is true that some hotels do not change the sheets between guests. This happens for a number of reasons, from it being a really scungy hotel to the maids just leaving a made bed, made. It's possible that a guest could have slept in the bed and remade it and the maid will leave it that way.
It is not illegal in all states to not change the bedding in between guests. Laws vary from state to state and municipality to municipality. And even where it is illegal, enforcement is another issue.
Essentially, the more you pay for your hotel stay, the better chance you have that it is clean and disinfected.
Also, higher end chains have started doing away with comforters, blankets and bedspreads and started using duvets and duvet covers which, in addition to the sheets are laundered in between guests without fail. And guests may ask they be laundered during one's stay as well.