Also, it's important to know why the general rule is to capture towards the center. This is because doing so increases your control of the center (this is why a c-pawn is slightly more valuable than a b-pawn, therefore ...bxc6 is somewhat of a promotion in general). Without actually knowing much about this variation, I'd imagine that the half-open d-file allows the black queen some center control, and ...dxc6 also allows the light square bishop to develop to better squares.

You should also look at this variation of the Scotch: 1. e4 e5 2. Nf3 Nc6 3. d4 exd4 4. Nxd4 Nf6 5. Nxc6 bxc6 6. e5 Qe7 7. Qe2 Nd5 8. c4 Ba6 (I think I got all the moves right...). Here, 5...bxc6 is preferable to 5...dxc6. From what I've found playing this and similar variations, black's light square bishop works best on a6 (as seen here). In other variations without 6. e5, having played 5...bxc6 allows an eventual ...d5.

In the DERLD, attempting to do so is a recipe for disaster, as the following variation indicates. As a result, Black can probably be forced to exchange his e-pawn with disadvantage almost as easily as in the Old Steinitz defence. Here is an example of how badly trussed up Black can get if he tries too hard to hold the e-pawn:

1 e4 e5 2 Nf3 Nc6 3 Bb5 a6 4 Ba4 Nf6 5 O-O Be7 6 Bxc6 bxc6 7 Nc3(!, *1) Nd7 8 d4 f6?(*2) 9 dxe5 fxe5 10 Be3 O-O 11 Qd3!(*3)

*1 - Probably a surprise, as in most Morphy variations c3 is indicated, making an early Nc3 subpar. Now Black has to actually guard the e-pawn. It's surprisingly difficult! Bd6 reduces the B to a P, f6 isn't possible at the moment and nor is Qe7. Probably best is Qd6 preparing to castle long, and accepting the loss of the strong point (as after 8 d4, Black cannot effectively reinforce the pawn at e5 and will have to exchange).

*2 - The weakness on a2-g8 doesn't seem like much, even though Black at this point is more or less committed to O-O due to the central tangle. WRONG! It's likely better here still to accede to exd4.

*3 - White will now continue with Rad1 enforcing his supremacy on the open file, and Black hardly has a reasonable means of development open to him. For instance b6 loses a Pawn to Qc4+ and even if it didn't, after 12 Rad1 Black cannot develop the LSB, as the Knight requires protection. Meanwhile, disentangling the Knight can almost only be done by imprisoning the DSB, due to the threats of Philidor's Legacy that occur along that dratted diagonal. The logical but fatal line is: 11 ... Re8 12 Rad1 Bf8?? (it's easy to think this is the road to recovery, the e-pawn is protected again, the DSB has not been forced to be immured, Qe7 can be played soon to get out from under and enable the Knight to move and the LSB to be free, etc. In fact, Black will be rudely awakened; at the least, some such prophylactic as 12 ... h6 is essential first, and even then Black will still have a cramped game where he is likely to become exposed to attack while still disentangling his pieces. The following diagram is the calm before the lightning ...)

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