I hope they don't , it seems kind of cheap.
It's not really.
It's an authentic medieval strategy, and one which would have legitimate medieval "engineers" involved.
And it simply means that you have to counter that, and there are plenty of ways to do that.
Moats filled with water make it difficult for people to mine to begin with, but if they do mine with the water there, it could make it impossible for them to set fire to the supports.
You could also build on top of a large stone area, rather than softer dirt. It would either be impossible for someone to get to a location to dig out such a rocky terrain, or it would mean that sapping took far too long to be an effective tool.
Or, you can build your castle in areas with useful terrain. Having your castle against a river/sea gives you a natural defence as no one can attack you from that side to undermine, or on a rocky hill/mountain. Or even in a more traditional motte and bailey design, you can create such a large earthmound that it'd be difficult to undermine the towers.
I don't think it's something they should avoid, because it's a historically accurate tactic, and facing those sorts of threats are things that medieval people had to think about when constructing their castles.