Hmmf. I see what you're saying. If I had $50k that I could afford to lose it sure would be fun to bat around the BTC market like a cat playing with a mouse, but this is similar to saying that what some hedge funds or other big institutional 'investors' have done in the past is OK too.
The result of both of these types of actions is clear - it makes the market for the smaller player unpredictable and dangerous to be involved in.
In one case, the small guy might lose or gain big on his BTC, in the second case, people might (and did) lose their houses.
Personally, I think these two should be classed as the same, and the guy paying with BTC is as bad as the actions of people in the sub prime mortgage crisis.
I don't mind people trying to manipulate the market. For a single, non-rich investor, this might be the one opportunity they would have in their life to have enough strength to move an entire market (legally, that is).
There's a symbiotic relationship here, where miners and those only interested in trading bitcoins are mutually beneficial. Traders, even vicious ones, give value and validation to the currency. Also, other v-words may be involved.