I really don't think it is a big issue at all regarding having the tv on & frankly if you are getting free babysitting, then you surely could make a few allowances for your parents watching a bit of tv.
I would really be more angry about the fact that they came & drilled holes in your house to secretly put an antennae up, that is beyond rude imo.
Trust me, as a sahm for the majority of the last 3 years, being at home all day watching kids can definitely be boring at times & frustrating. We usually have the tv on here in the morning at breakfast, then it goes off & we play (bikes, playdough, drawing etc..), it is then on again while lunch is being eaten & then kids have their naps. In the afternoon we might go for a walk or drive up to the shops. think that tv in moderation or for a bit of background noise is ok, I def wouldn't use it as a babysitter all day though (although if I need to make dinner & the kids are being ratty, I will usually pop the tv on for 30mins!).