In principle, however, universal coverage isn't an insurance scheme. The only reason the US is stuck in the insurance trap is because they want a free market solution. Insurance is the ugly child of free market medicine. Fewer people go for prevention, insurance attempts to not pay out, premiums are only semi-competitive, care is stratified, etc. Even when it is working the way it 'should', it fails.
So, people supporting the insurance scheme for medicine are simply working within a fundamentally flawed system... but they are limited by the politics and people's ideology as to what changes can be made.