Yes, it is common for ISPs and email service providers to store your password in plain text, or a format which is easily recoverable to plain text.
The reason for this has to do with the authentication protocols used with PPP (dialup and DSL), RADIUS (dialup, 802.1x, etc.) and POP (email), among others.
The tradeoff here is that if the passwords are one-way hashed in the ISP’s database, then the only authentication protocols that can be used are those that transmit the password over the wire in plain text. But if the ISP stores the actual password, then more secure authentication protocols can be used.
For instance PPP or RADIUS authentication might use CHAP, which secures the authentication data in transit, but requires a plain text password to be stored by the ISP. Similarly with the APOP extension to POP3.
Also, all of the various services which an ISP offers all use different protocols, and the only clean way to have them all authenticate to the same database is to keep the password in plain text.
This doesn’t address the issues of who among the ISP’s staff has access to the database, and how well it is secured, though. You still should ask hard questions about those.
As you’ve probably learned by now, though, it’s almost unheard of for an ISP’s database to be compromised, while it’s all too common for individual users to be compromised. You have risk either way.
See also Am I wrong to believe that passwords should never be recoverable (one way hash)? on our sister site IT Security