Of course they are enforced after you leave but only under certain circumstances. Once payment stops you have been laid off and that is when non-competes stop. USSC ruled that the company then no longer finds value in your services and non-competes fall apart. Non-competes usually accompany severance packages that stipulate continued non-compete agreements, but when you are laid off they company has no grounds to enforce the agreement. BTDT, got the t-shirt.
Looks like United is trying to retain their employment without paying them.