http://www.fontsquirrel.com/fonts/Museo-Slab
Generally we encourage people to notify us if font licenses change. Which they sometimes do. Which is a shame.
The whole font licensing business is crap, I can agree to that.
Considerably better rates than myfonts.com but still not free-gratis nor free-libre.
Their license is here: http://www.fontspring.com/fontface. You are required to use an alternate rendering of the font with all open-type parts removed that has been subset to the usage required on the site. This makes it quite easy for them to check if you're infringing they just download the font from your site and make sure it meets the license requirements, if not then they can sue you with impunity.
They can probably argue that if the domain holder on whois isn't the licensee then you're also infringing (see license term 6).
In short I don't think the copyright holder for Museo Slab offers their font/typeface for free and suspect anywhere that is offering it or using it is infringing copyright.
I chose Museo Slab as an exemplar, this is probably also true of the other fonts on that site.