It is positive action and it is lawful under the Equality Act. The MP who is complaining about it is a member of the party that was in government when the Equality Act came in.
Positive action is lawful if it can be shown that it "enables or encourages people who share a protected characteristic to participate in an activity in which their participation is disproportionately low." So as long as it can be shown that historically BAME participation is disproportionately low at events like this it is legal and legitimate. I believe young people are also being charged less.
The Resolution Foundation found, in research from last year, that BAME families earn upto £8900 per year less than their white British counterparts.
Austerity/spending cuts also has a disproportionate impact on BAME according to many reports.
So we have a group of people who share the same characteristics, who reports show have less disposable income than their white counterparts and are likely to be disproportionately represented at events like this....and they are offered an incentive to attend and it has been criticised by a Tory MP, Andrew Bridgen, who while an MP was paid over £7000 per month by his own firm for 6 hour work a month while at the same time calling for a pay increase for MPs.