It's Labour policy (UK-wide) to criticise profits from rail franchises, now nationalisation is in the official manifesto.
Among other things it's designed to scare bidders off so the franchises will fall into government hands.
They used the same tactic in 1996 and the result was the Treasury got less for the sale of franchises, rolling stock and the rest of BR than they should have done ("fire sale", management buyouts etc).
Arriva (any franchisee) needs to make a profit if the owners are to keep investing. Something around 5% is not "excessive".
If the ATW profits are higher than average, that's down to the small print of the franchise agreement, and the W&B one may well have been lax, with DfT/WG locked in for 15 years (by the SRA).
That's not ATW's fault. Franchises have got cleverer since, but also the bidders are more wary.
Next time, expect WG to sign a franchise agreement with a tight operational spec, at least initially.
However the WG wish list is so long, and their plans so vague that there are bound to be areas where the contractor will have a field day on cost recovery (Metro, new trains etc). It's called a "get well" contract.
WG also want to go for a 15-year franchise again, partly to avoid the cost/bother of going through the process again after 7 years.
You get what to asked for...