I don't know for sure, but I was curious so I Googled it, and quite a few results came up. It seems to come down to one of two, or maybe both, possibilities. The information I found related to Federal Champion ammo. The first reason given was that they use thinly-plated bullets, and apparently it's possible for a piece/strip of the plating to come off the bullet as it passes the port and get flung back at the shooter. Some people suggested that this occurrence was rare, but Federal put it on their boxes for liability reasons. The other was that the ammo was low-powered, and with a ported/compensated barrel it could negatively affect reliability.

I have no idea how accurate this is, but it's what I was able to find. If you really want to know, it might be worth the trouble to contact Federal and ask them.

Hope that helps.