Actually, one of the appeals of PDX is that their games are largely DRM free, as they believe fans should not be disadvantaged with cumbersome DRM just because they want piracy groups to spend an extra 10-15 minutes cracking their game. If I recall correctly, this is the official position of their CEO, and they do mention this time and time again, likely for PR reasons. It definitely wins browny points with me though.
From a business perspective, it is just not as worthwhile for smaller game companies and publishers to invest in anti-piracy measures. Anti-piracy software is an investment that is supposed to, in theory, influence the 'swing' pirates to purchase the game. No one is really fooling themselves that DRM will make piracy impossible, but the idea is that there may be pirates who are hindered from pirating thanks to DRM, and thus, with no other alternative, would go on and purchase the game. The number of 'swing' pirates is I would, mostly theoretical and open to debate. Certainly though, given the financial and PR costs of DRM, it would be a less viable option for smaller companies than larger companies, where the number of 'swing pirates' are higher due to the overall larger customer base and where the financial cost of DRM is proportionately less due to greater total revenue.
Just to give an example:
PDX is a small publishing company compared to, say, EA. PDX is about 0.5% the size of EA when it comes to revenues. So, (just examples here) if EA has 100,000 customers, then PDS would have have about 500 customers (0.5%). If the percentage of people who would be persuaded to buy the game if DRM is implemented is 1%, then investing in DRM would bring EA 1000 customers, but it would only bring PDX 2-3 customers. The cost of investing in DRM is the same for both companies, and EA has larger overall budget. Under this scenario, it makes sense from a business perspective for the larger company to embrace the implementation of DRM and the smaller company to shun it.
Of course, this is all assuming that 'swing pirates' are around 1% which may not be entirely true. There is certainly the argument that DRM may actually lower the customer base or even encourage piracy... But I'm guessing good ol' EA has hired people who has conducted professional analysis...
EDIT: fixed calculation errors