What the priority and severity of the bug means about?

• In form * symbol is missing for mandatory fields.
• The fields accept invalid data. 
• Validation message doesn't get displayed properly. 

Yes.,Above listed things are the validation issues, those issues doesn't get much severity or priority in developer's concern some times. Above are not a bugs in application? But why it doesn't get much attention.
Consider the Registration form in one ABC application., If that form nor properly validated means, then could not able to receive the real and correct input form the users.
'Email" field exists in the same form, and once user register with the application, conformation mail sent to appropriate user's mail. This is the functionality, and perhaps the developer had no care about validation while code this registration form and forgot to code mandatory option for 'Email' field.
The same bug raised by tester but its priority was low all because, it comes under validation category, application was released.
And user came and view the application, wish to register with the same.User enter her details in registration form, and you know this user was very clever and decided like why do i give my info which are all not mandatory..? so let me fill this form with the mandatory fields, so the user not enter her email and submit the form. As usual the user got an beautifully designed Thank You note page and the message like you will get the conformation mail shortly.., :) Alas! User is clever but yet not able to sign up with the application. Not even this one  those who all other cleverers submit the form with out their Email address not able to sign up with the application.
Morel of this story: We can prioritize the bug belong to validation category as low, but the cause and effect of same bug exists in application is huge. If the tester consider the risk of this bug means the same should tagged with the Very High severity. Am i right? :)