Like most network music radio feeds, MeTV has a few breaks per hour where stations can play spots, and if the station has no spots, the station can fill the breaks with a timed song and station identifiers and/or jingles. This is done locally by programming automation to fill the breaks.
I've only very occasionally heard any local advertisers on WMEX during the MeTV broadcast hours, and MeTV itself rarely inserts advertisements into the radio network feed, instead they promote their bread and butter, the MeTV television network that airs classic and vintage TV programs, which is full of ads on television. The radio network serves them to promote their TV network.