Bad practice? Hell no, as a former mod on a very high end fashion forum with over 200,000 members the new member rules here are awesome. The only rule we had for new members was a 15 post minimum for private messages. Let me tell you that as a mod it was a huge PITA. Especially since selling,advertising your own ebay auctions etc was ban worthy. We had to rely on the mods eagle eye or reported posts by other members in order to catch spammers who would join, spam, get banned, come back under a different nic,spam some more.
Not all forums are the same, some are more lax on the restrictions than others but the most important thing to remember is that the rules are set by the Admin of the respective forum.