Free Software I write is is strictly the result of selfish motivations -- I want to grow my own knowledge of some system or I need some tool or functionality that does not exist (or I cannot afford to buy).
Satisfying those goals does not have to conflict with my day-job. If, however, I want to author some system that may conflict with the interests of my employer, I either need to
a) clear it with my employer -- it's not fair for them to pay for their competition
b) forego the project until there is no ethical conflict with my day-job
So, this only really becomes a problem when a developer wants to write software that conflicts contractually (or ethically) with their bread-and-butter.
This is a situation where some developers want to have their cake and eat it too -- they want to get paid for their work, but they want to compete with the people who are paying them for their work.
Yes, software should be free (as in speech), but not at the expense of someone who doesn't believe in the cause.