I’ll let Diego give an official response, but I’ll relay what I was told.
Source: New Exchange Rate Mechanism - #29 by bjoernek
When the weights went back to one there was still a small incentive to spam because there are a lot of exchange rate proposals and those have a .01(x144 per day…so you start with a total weight of 1.4 for all DFINITY followers on Exchange Rate) weight. So there was still an imbalance with the weights at 1 for all others. If one had issued a bunch of spams every day you could have eventually gotten to where there would have been an advantage.
The problem was about to get worse though because the exchange rate proposals are going away. So all of that weighting that was counterbalancing spams is going to go away. There was especially going to be a problem on weekends when there were going to be none of the standard subnet and upgrade proposals that usually balance out the weightings.