That might be true for console titles, but steam based games are a little different, due to the possibility of steam sales. By using the advantages of digital distribution (e.g. infinite stock, razor thin overhead) steam makes it possible to have sales of older games at huge discounts. This then generates a huge volume of sales which can generate substantial revenue (to the degree where steam-sales have actually rescued some game companies from the brink of extinction). Valve doesn't have to release sales figures, but it's a good bet that the revenue curve over time for TF2 has a much fatter and longer tail than for your typical AAA console title.

That being said. The advantage of making TF2 free-to-play are huge. Firstly, the hat-conomy generates plenty of revenue on its own. More importantly, TF2 could quite easily become a gateway drug for casual PC gamers to use steam. More users playing TF2 means more users looking at, say, Terraria for 10 bucks, or Left 4 Dead 2 for 20, or some other relatively recent game at a ridiculous price during a sale.

In my experience vanity items with no effect on gameplay attract droves of people with to much disposable income. When I was still playing WoW lots of people were buying things like pets and mounts with no benefit other then looking cool and showing off. I don't see how TF2 hats will be any different.

Regardless, as people said the sales of TF2 have probably mostly dropped off, distributing the game for free costs nothing (well, it costs hosting and bandwidth but I don't suspect those costs make up any significant percentage of Steam's total costs, so effectively the hosting is free) so they have barely any income to lose and a massive fanbase and ingame transaction economy to gain.