Sub-committees certainly aren't illegal. ECs can take advice from whomever they want but only the members of subcommittees who are EC members have a vote.
Smart ECs use sub-committees (comprising at least one EC member and any non-EC members as are interested) as a way of spreading their knowledge base and introducing potential future EC members to the ins and outs of the EC world. Only those who feel threatened by other people's opinions would have any reason to object to sub-committees.
On this very point, the chairman of my building was recently fulminating that outsiders' opinions were “more trouble than they're worth” when someone pointed out that the last sub-committee approved by the EC had dealt with a gardening issue very efficiently … and (oops!) it had included his wife. Much hilarity ensued.
It really depends what you want in your EC – a lively exchange of (hopefully) informed opinions or a cluster of know-alls whose views – informed or otherwise – dare not be challenged.
Regarding the Act, there's nothing in it that I know of to cover sub-committees but then there is no need for any legislation. They're not getting paid, they don't have any specific voting power and their views can only be expressed via EC members or, in the case of non-EC members – at the invitation of the EC.
Sounds like sour grapes from someone whose opinions have been over-ruled.