I believe it is because of two reasons they say that.
#1) They are being ultra conservative.
#2) I think I read somewhere that there was an earlier version of the hubs that was not as strong. They have since upgraded. Sometimes I see that disclaimer printed and sometimes I see an ad with no disclaimer at all.
I did quite a bit of homework before deciding to go with the small (5 on 4.5") hubs. There were lots of guys running 35" tires with no problems with these hubs. With alloy shafts, the weak link is either the u-joint if the wheel is turned and in a high stress situation, or the hub. I want it to be the hub. So far I had no problems with the hubs with 33's for 6 months and no problems with 35's for 2 trail runs. I carry spare hubs.