The blocks on port 25 usually aren't about prohibiting a person from using different ISPs for inbound and outbound traffic (though they do have that effect) so much as they are to prohibit people from running their own mail servers, often without their knowledge.
Nearly all spam these days is funneled through virus-infected computers. It's one of the ways that virus makers make money, and it's part of the reason computer malware is a huge and highly profitable business.
One of the first viruses to do this was a bit of malware called W32/Minmail.MM. It did something new: on infected computers, it set up a mail server running on port 25. Then, it reported the IP address of the infected computer back to the virus creators. The virus creators would then sell or rent time on the infected computer to spammers, who would use it to funnel spam without the computer owner even knowing it--all the computer owner would know is that his computer was acting slow.
Almost all modern malware will set up mail servers as part of what it does. Blocking port 25 was a technique to combat spam from infected computers, though these days modern malware sets up mail servers on a number of different ports, and/or use authentication, encryption, or both to hide the mail traffic.
