Mastery is a measure of skill, not a tool for additional discipline.
Preventing already disciplined players from earning Mastery 6 & 7 through the Hextech restriction, is adding more punishment for seemingly no reason.
They have already served their prison term, paid their debt & are on parole (progressive discipline if they continue to violate the summoner's code).
The fact they can earn Mastery points for levels 1 - 5 but are stopped at 6 & 7 suggests this is a RIOT oversight due to the Hextech restrictions.
If it's not an oversight, then the inconsistent messaging of Mastery 1 - 5 is OK but NO to 6 & 7 really dilutes the intended behavorial changes that discipline aims to achieve.
99.99% of the time, disciplined players deserve punishment... but that doesn't mean they are the scum of the earth. Mastery 6 & 7 restriction is a step too far.