DoTs can scale above 100%.
The reason this weapon damage cap was introduced was because of a combination of two issues:
- Resist Reduction was scaling above 100%
- Resist Reduction was affecting enemies’ resistance to Health Reduction
So players could get a good deal of RR on weapons, use Shadow Strike to multiply it by 5, and then deal 90%+ of a boss’s health per blade of Phantasmal Blades.
As to lifesteal, it changed in August 2015 with the B27 patch:
- % Attack Damage Converted to Health bonuses have been reduced, and in some cases removed/replaced, on all equipment. In addition, it no longer scales above 100% Weapon Damage on skills (ex. 20% life steal with 200% Weapon Damage would still be 20%, not scale up to 40%).