I'm going to assume that we're talking about a country with global healthcare here, but either way it's not restricting your freedom in any sense of the word - it's ensuring that high risk individuals pay more into a system they are more likely to take out of. Your freedom isn't restricted in any way, you're just going to have to pay more to enjoy your potentially dangerous activities. In a private system like that of America I'd argue that it'd be an unfairly placed tax, since obesity doesn't harm anyone but the obese.
In Canada, however, I can understand why people would want me to pay more taxes on cigarettes and alchohol since they increase the chance I'll be accessing the universal pool of health care resources. Likewise, I have no problem with an obesity tax for that reason. Lets not break the bank and reduce the quality of service for everyone because some people feel the need to scarf pork rinds and sit on their asses all day.
As for fat people losing their jobs because the HMOs demand it, I don't know how you'd want to regulate that other then government intervention. They're a business and covering obese people is bad for their bottom line.