They could stop allowing that sort of behavior, but they know that a large part of their user-base consists of 'self marketers' and other less subtle spam.
Why can't I sort or filter any of my incoming tweets in any meaningful way? I can have favorites. I can make lists. I CAN'T just type in 'Fck', and stop my feed from showing me the tweets with the word 'Fck' in them.
I can search for tweets about 'Ruby On Rails', but I can't just add those people to a list called 'Ruby On Rails' without clicking each one of them... Several times.
To top it off their API is so limited that just retrieving a list of the people I'm following means I get blocked for an hour before I can do anything with it, and bypassing their silly API is against the TOS. So writing my own tool to filter properly is straight out.
I wanted to write a simple naive Bayesian filter app to separate the wheat from the chaff for me, but they make it damned near impossible to do so without being sued.
In no way is that not your fault. None whatsoever.
I do agree that a filtering mechanism would be MUCH appreciated, however.
Let me restate it more precisely: The fact that my needs are not being met is their fault. The fact that I have those needs is mine.
My use case, and those of many others, requires the ability to 'follow' others and still filter their 'useful' postings from their 'non-useful' postings. This is 'my fault', in that I require more from the service than they are either willing or able to provide at this time.
Twitter has reduced my ability to use the service, in the way that I would most like to, by limiting their API and preventing screen scrapers from functioning in a legal and non-harmful way. None of this is any way my fault.
I still think my original phrasing was more concise, if not as explicit.
I do think, though, that not reciprocating is probably sufficient. I should've mentioned that I'm also a minimalist and like to keep my lists clean.
I have a pretty short twitter name and get @ed with garbage messages by random people.