Hi Fab,
those are just standard transparent rubbery pads... they offer a non slip type grip. The speaker is not fixed or attached to the stand in any other way. It seems like the speaker is too heavy and is wide to need any additional attachment to stand. I tried to gently push it to see if would easily topple. due its weight, it actually feels like it is attached to the stands. Not likely to topple easily. These stands also have a very wide base which make it quite stable overall. if you are worried about it falling over... even if it were fixed to the stands ... it could just topple along with the stands.
-roshan