I've noticed a few of these as well. One possibility is that the station is either blacking-out commercials, due to contractual requirements, causing a minute or two of silence, before the main program comes back on. Another possibility is that certain programs in their entirety are prohibited from being included on a station's internet stream, due to contractual requirements or because the program streams from its own separate website, sometimes on a paid subscription basis. Major League Baseball games are (supposed to be, and usually are) blacked out from their affiliate stations' internet streams, as are some syndicated talk show hosts (like Jim Rome and Michael Savage). When this is the case, sometimes the station's stream plays repeats of other programming and sometimes it plays dead air. Checking back after an hour or two can find that a live stream has returned.
Otherwise, it's also possible that the stream url is obsolete, but still playing a blank feed. Checking WBZ specifically, the Radio Roku database has five different urls that it goes through when trying to stream the station:
http://provisioning.streamtheworld.com/ ... DIALUP.pls
http://provisioning.streamtheworld.com/ ... DIALUP.asx
http://tuner1.dc1.sonixtream.com/playli ... zWBZAM.asx
mms://bdcast-infinity-wbz-am.wm.llnwd.n ... ity_WBZ-AM
#1 and #2 work fine, as they are the two new url formats recently adopted by CBS stations nationwide. #4 shouldn't even be there, since it's the station's main website. #3 and #5 are obsolete CBS url formats that will never be used again, but interestingly, #3 plays a short 64 kbps advertisement fragment that could perhaps prevent feeds #1 and #2 from playing. In this case, it's probably a good idea to purge #3, 4 and 5 from the database.