But what stops them from reselling it or extending it themselves?

Honesty. What stops you from running of copies of Gwen Stefani's latest CD, except honesty and good taste?

Is this [what] the so called obfuscation used for?

Sometimes, but obfuscation seems to be more a form of amusement.

I wonder if this has something to do with the license of the software which states that any one can freely distribute it and extend it.

No. The license does not apply to programs written in Perl; it applies to Perl and its core modules. Similarly, programs written in gcc or g++ are not subject to the GPL, but gcc and g++ are.

Even compiled binaries can be de-compiled. It's what disassemblers are for.