A Thought on Flag Waving

With the recent protests against immigration controls by illegal (and occasionally legal) immigrants, there's been a lot of flag waving, by illegals waving the Mexican flag.

So, when Americans wave American flags why is it suddenly offensive speech?

But more importantly, where are the folks who immediately jump up and get excited defending the rights of folks to burn the flag? Why are they not now also defending the right of free speech in waving a flag?

Can someone explain the silence and the difference, especially someone who has supported the ACLU (conspicuously silent so far) previously?