Because the healing they dislike is uninteractive healing, not interactive healing.
Uninteractive healing is healing gained passively, without possible interaction from the opponent. Things like Soraka's W can be very much like this, if Riot hadn't forced interaction into her kit (Adding a health cost to her W, which requires her to land her Q on an opponent to be able to continue healing).
Interactive healing is healing that is not uninteractive. Autoing a minion is not passive, and enables the opponent to interact with you in some way. Therefore, lifesteal is not uninteractive (Albeit not very skill-intensive), and is therefore acceptable so long as it is kept in reasonable numbers.
Riot has also, in the past, show willingness to make exceptions to previous rules when following that rule would create more issues than it would solve. Lifesteal is an essential stat for DPS champions in general, but especially so for marksmen, since they simply would not survive long enough to kill a tank without it.
Third thing: Life steal is still post-mitigated damage. Not pre-mitigated. Post-mitigation refers to actual penetrating damage - damage after armor, MR, damage reduction, invulnerability, or damage blocking. You heal less on an auto against someone with 400 armor than someone with 100 armor. You heal nothing against an invulnerable/unkillable target. Just because you heal off of excess damage dealt as well as normal damage does not mean it's pre-mitigation healing.