Yes, you can use them commercially, and even include them within a product that is sold commercially. Usage and redistribution conditions are specified in the license. The most common license is the SIL Open Font License. Some fonts are under the Apache license or Ubuntu Font License. You can redistribute open source fonts according to those conditions.
Yeah:
https://developers.google.com/fonts/faq#can_i_use_any_font_in_a_commercial_product
As an American, I agree with this message 100%.