Slavery, as referenced in the 13th amendment, is a narrow legal term which does not encompass such things, unfortunately.
If that were the case, taxes would be slavery as well - an argument made by many Liberal economists, (such as my namesake). Unfortunately we do not, as a society, think this way.
Taxes is forcing citizens to pay for society, even as much as we don’t want to.
This healthcare bill is forcing us to pay a third party/entity (health insurers) under threat of loss of freedom thereby ‘enslaving’ us to another entity. That is a huge difference.