I can't speak for all the double-negative posters, or even for everyone in the Locus of Awesome, but here's why I think it's a natural connection:
Restrictions on freedom are not exclusive to government. People's practical choices are limited by their savings/debts, earning potential, education, health, free time, etc. The "restrictions in the economic sphere" are generally designed to give more choices or opportunities to people who would otherwise have very limited options.
Child labor laws, the minimum wage, the 40 hour work week, workplace safety laws, environmental regulations, publically funded universities, minimum housing standards, automobile safety regulations, the FDA, fire codes, etc. all put limitations on the actions of corporations or capitalists (in the
first sense of the word). Even programs that don't directly affect capitalists (free, mandatory, primary and secondary education and publically funded health care) restrict them economically if they are paid for by a progressive tax system.
However, these laws remove practical restrictions for many people. I'd argue that more people benefit from increased freedom as a result of these laws than people suffer from decreased freedom. I'd also argue that the people being restricted still have way more options than most people, and won't feel the sting too badly.
Even though I said I couldn't speak for my fellow libbies, I'll give it a quick shot: I think the natural tendency for a liberal, or for a US citizen who votes Democratic is to identify with The Little Guy, against the Fat Cats. They don't want the CEO's dictating the distribution of wealth and screwing over hard-working Joe Schmo, and they don't want senators dictating where Joe can and can't stick his penis.