If you make a derivative work for your own private/personal use, there's no problem. If you distribute an unmodified copy (no alterations), that's also OK. But when you make a derivative work and and distribute the result (such as selling a modified version of pfSense pre-installed on hardware) at that point it's, a new product.
http://www.linuxfoundation.org...
"A trademark should not be used as part of your product name."
https://www.freebsdfoundation....
"3. If we grant you permission to use the Marks, your use of the Marks must always be fully and clearly reproduced, and you may not incorporate any of our Marks into the trademarks, service mark, logos, name of your business, project, organization, or username, unless you have the express prior written permission of the Foundation."
The pfSense CLA and such closely mirror that of the Apache product. Here is what they say on http://www.apache.org/foundati...
"This License does not grant permission to use the trade names, trademarks, service marks, or product names of the Licensor, except as required for reasonable and customary use in describing the origin of the Work and reproducing the content of the NOTICE file."
The confusing part is that people seem to mix up distributing an unmodified copy (which is OK to put on hardware for sale, so long as the mark is respected) with distributing a modified copy, which they may not realize is now a derivative work and thus violates the trademark. People interpret that as being told they can't sell the software, but what they can't do is sell their own derivative work and call it by someone else's trademark. (See above example, re: Coke)