The large supplies of group funds found in these, ready to be abused, are a ticking timebomb for the economy in terms of inflation. An automatic one time removal of robux inside groups which do not have owners would eliminate this deposit of robux from flooding into the economy, avoiding resulting inflation and illegitimately gained riches.
People have already been getting extreme numbers of robux from this, the thread linked below being an example -
http://www.roblox.com/Forum/ShowPost.aspx?PostID=163269578
Resulting inflation easily leads to increased limited prices, increased DevEx expenditure for Roblox and a bad effect on TC rates - it's something which negatively affects everyone except those abusing the feature.