[cwp]

First, not all free licenses require you to share changes. See, for example, the MIT and BSD licenses. They only require that you preserve the copyright notice, and refrain from suing the original authors.

Second, even the GPL doesn't stop you from using a modified version, just from distributing modified versions without the source code. From the preamble to the GPL "if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received."

The fact that web services don't actually distribute software at all is what inspired the AGPL license in the first place.

Finally, "open/free" refers to the distinction between "open source" licenses as defined by the Open Source Initiative (opensource.org) and "free software" as defined by the Free Software Foundation (fsf.org). As a practical matter, they are similar, but the two organizations have different philosophies.