They're only doing it because they know ME2 isn't worth £5 now. Not with all the DLC you'd have to buy and the fact an 'ultimate edition' is almost certainly right around the corner for full price anyway.
Still, I just bought ME on steam, so to get ME2 for free is a nice surprise.
I got DA2 using a rainforest voucher that I won for filling in a questionnaire anyway.
Cheers for the heads up